System.Xml.Serialization.XmlSerializer.FromMappings throws NullReferenceException using a pre-generated serialization assembly

### Description

Hi, we are generating some .net standard 2.0 library via a WSDL file using `dotnet svcutil`. Also, we are generating serialization assembly using `dotnet-svcutil.xmlserializer` to speed up serialization.

Some details about projects:

VimService.csproj

```csproj
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="System.ServiceModel.Duplex" Version="4.5.3" />
    <PackageReference Include="System.ServiceModel.Http" Version="4.5.3" />
    <PackageReference Include="System.ServiceModel.NetTcp" Version="4.5.3" />
    <PackageReference Include="System.ServiceModel.Security" Version="4.5.3" />
  </ItemGroup>
</Project>
```

VimService.XmlSerialization.csproj

```csproj
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <DotNetCliToolReference Include="dotnet-svcutil.xmlserializer" Version="1.0.0" />
    <PackageReference Include="System.ServiceModel.Primitives" Version="4.5.3" />
  </ItemGroup>
  <ItemGroup>
    <ProjectReference Include="..\VimService\VimService.csproj" />
  </ItemGroup>
</Project>
```

So, we use these libraries to interact with the VMWare vSphere in .NET 6 console app.
Unfortunately, we catch an exception on object serialization:

```
System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Xml.Serialization.XmlSerializer.FromMappings(XmlMapping[] mappings, Type type)
   at System.ServiceModel.Description.XmlSerializerHelper.FromMappingsViaReflection(XmlMapping[] mappings, Type type)
   at System.ServiceModel.Description.XmlSerializerHelper.FromMappings(XmlMapping[] mappings, Type type)
   at System.ServiceModel.Description.XmlSerializerOperationBehavior.Reflector.SerializerGenerationContext.CreateSerializersFromMappings(XmlMapping[] mappings, Type type)
   at System.ServiceModel.Description.XmlSerializerOperationBehavior.Reflector.SerializerGenerationContext.GenerateSerializers()
   at System.ServiceModel.Description.XmlSerializerOperationBehavior.Reflector.SerializerGenerationContext.GetSerializer(Int32 handle)
   at System.ServiceModel.Description.XmlSerializerOperationBehavior.Reflector.SerializerStub.GetSerializer()
   at System.ServiceModel.Description.XmlSerializerOperationBehavior.Reflector.MessageInfo.get_BodySerializer()
   at System.ServiceModel.Dispatcher.XmlSerializerOperationFormatter.SerializeBody(XmlDictionaryWriter writer, MessageVersion version, String action, MessageDescription messageDescription, Object returnValue, Object[] parameters, Boolean isRequest)
   at System.ServiceModel.Dispatcher.OperationFormatter.SerializeBodyContents(XmlDictionaryWriter writer, MessageVersion version, Object[] parameters, Object returnValue, Boolean isRequest)
   at System.ServiceModel.Dispatcher.OperationFormatter.OperationFormatterMessage.OperationFormatterBodyWriter.OnWriteBodyContents(XmlDictionaryWriter writer)
   at System.ServiceModel.Channels.BodyWriter.WriteBodyContents(XmlDictionaryWriter writer)
   at System.ServiceModel.Channels.BodyWriterMessage.OnWriteBodyContents(XmlDictionaryWriter writer)
   at System.ServiceModel.Channels.Message.OnWriteMessage(XmlDictionaryWriter writer)
   at System.ServiceModel.Channels.Message.WriteMessage(XmlDictionaryWriter writer)
   at System.ServiceModel.Channels.BufferedMessageWriter.WriteMessage(Message message, BufferManager bufferManager, Int32 initialOffset, Int32 maxSizeQuota)
   at System.ServiceModel.Channels.TextMessageEncoderFactory.TextMessageEncoder.WriteMessageAsync(Message message, Int32 maxMessageSize, BufferManager bufferManager, Int32 messageOffset)
   at System.ServiceModel.Channels.TextMessageEncoderFactory.TextMessageEncoder.WriteMessage(Message message, Int32 maxMessageSize, BufferManager bufferManager, Int32 messageOffset)
   at System.ServiceModel.Channels.MessageEncoder.WriteMessage(Message message, Int32 maxMessageSize, BufferManager bufferManager)
   at System.ServiceModel.Channels.BufferedMessageContent.EnsureMessageEncoded()
   at System.ServiceModel.Channels.BufferedMessageContent.TryComputeLength(Int64& length)
   at System.Net.Http.HttpContent.GetComputedOrBufferLength()
   at System.Net.Http.Headers.HttpContentHeaders.get_ContentLength()
   at System.Net.Http.SocketsHttpHandler.ValidateAndNormalizeRequest(HttpRequestMessage request)
   at System.Net.Http.SocketsHttpHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpClientHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpMessageInvoker.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpClient.<SendAsync>g__Core|83_0(HttpRequestMessage request, HttpCompletionOption completionOption, CancellationTokenSource cts, Boolean disposeCts, CancellationTokenSource pendingRequestsCts, CancellationToken originalCancellationToken)
   at System.ServiceModel.Channels.HttpChannelFactory`1.HttpClientRequestChannel.HttpClientChannelAsyncRequest.SendRequestAsync(Message message, TimeoutHelper timeoutHelper)
   at System.ServiceModel.Channels.RequestChannel.RequestAsync(Message message, TimeSpan timeout)
   at System.ServiceModel.Channels.RequestChannel.RequestAsyncInternal(Message message, TimeSpan timeout)
   at System.Runtime.TaskHelpers.WaitForCompletionNoSpin[TResult](Task`1 task)
   at System.ServiceModel.Channels.RequestChannel.Request(Message message, TimeSpan timeout)
   at System.ServiceModel.Dispatcher.RequestChannelBinder.Request(Message message, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs)
   at System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(MethodCall methodCall, ProxyOperationRuntime operation)
   at System.ServiceModel.Channels.ServiceChannelProxy.Invoke(MethodInfo targetMethod, Object[] args)
   at generatedProxy_1.RetrieveServiceContent(ManagedObjectReference )
   at VimApi.VimPortTypeClient.RetrieveServiceContent(ManagedObjectReference _this)
   ...
```

If the application is compiled under .net 4.8, then everything works well. Without using the pre-generated serialization assembly, everything also works well on .net 6.

Comparing the source codes .net 4.8 and .net 6, I've noticed that an additional check was added in #58932, which throws an exception. `serializers[i]` is null here:

```C#
public static XmlSerializer[] FromMappings(XmlMapping[]? mappings, Type? type)
{
    ...
    XmlSerializer[] serializers = new XmlSerializer[mappings.Length];
    for (int i = 0; i < serializers.Length; i++)
    {
        serializers[i] = (XmlSerializer)contract!.TypedSerializers[mappings[i].Key!]!;
        TempAssembly.VerifyLoadContext(serializers[i]._rootType, type!.Assembly); //added in #58932
    }
    return serializers;
    ...
}
```

There is no any verifying in .net 4.8, although expression `(XmlSerializer)contract.TypedSerializers[mappings[i].Key!]` also returns null.
Moreover, the values of the input parameters of the method `XmlSerializer.FromMappings(...)` are identical in both frameworks in debug, as well as the code in the serialization assemblies generated via SGen under .net 4.8 and via `dotnet-svcutil.xmlserializer` under .net standard 2.0.

Any advice, please? Workarounds?

### Reproduction Steps

Here are project sources, generated DLLs and source wsdl files.
[WcfCookieTest.zip](https://github.com/dotnet/runtime/files/8584062/WcfCookieTest.zip)

WDSL file for util `dotnet svcutil` is vimService.wsdl

### Expected behavior

No exception in this case

### Actual behavior

Exception is thrown

### Regression?

_No response_

### Known Workarounds

_No response_

### Configuration

dotnet info

```
.NET SDK (reflecting any global.json):
 Version:   6.0.201
 Commit:    ef40e6aa06

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19042
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\6.0.201\

Host (useful for support):
  Version: 6.0.4
  Commit:  be98e88c76

.NET SDKs installed:
  2.1.518 [C:\Program Files\dotnet\sdk]
  5.0.400 [C:\Program Files\dotnet\sdk]
  6.0.101 [C:\Program Files\dotnet\sdk]
  6.0.201 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.22 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.22 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.18 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 6.0.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 6.0.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.22 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.18 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 6.0.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 6.0.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 6.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.18 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 6.0.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 6.0.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
```

### Other information

_No response_

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System.Xml.Serialization.XmlSerializer.FromMappings throws NullReferenceException using a pre-generated serialization assembly #68659

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System.Xml.Serialization.XmlSerializer.FromMappings throws NullReferenceException using a pre-generated serialization assembly #68659

Description

Description

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Expected behavior

Actual behavior

Regression?

Known Workarounds

Configuration

Other information

Metadata

Metadata

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