`null` initializer for non-nullable property (or field) should warn

Similar to https://github.com/dotnet/roslyn/issues/26626
```C#
public class C
{
    string x { get; set; } = null; // expect a warning
}
```

Found in nullable dogfood.

Note: there are some IDE tests (in `CSharpDeclareAsNullableCodeFixTests`) for this bug, which should be unskipped. 