[Mono] Fix ParameterInfo.Name to return null for unnamed parameters

Summary

Fixes runtime inconsistency where Mono's ParameterInfo.Name returns empty string ("") for unnamed parameters while CoreCLR correctly returns null.

Problem

Failing Tests:

• OpenApiOperationGeneratorTests.ThrowsInvalidOperationExceptionGivenUnnamedParameter
• RequestDelegateFactoryTests.CreateThrowsInvalidOperationExceptionGivenUnnamedArgument

Root Cause:

// CoreCLR behavior (correct)
var param = lambda.Compile().Method.GetParameters()[0];
param.Name == null  // true

// Mono behavior (bug)
param.Name == ""    //  empty string, not null

ASP.NET Core validation checks if (parameter.Name is null) - this works on CoreCLR but fails on Mono.

Reproduction

using System;
using System.Linq.Expressions;

var unnamed = Expression.Parameter(typeof(int));
var lambda = Expression.Lambda(Expression.Block(), unnamed);
var param = lambda.Compile().Method.GetParameters()[0];

// CoreCLR: param.Name is null 
// Mono: param.Name is "" 
Console.WriteLine($"Name is null: {param.Name is null}");

Test Results:

• With only loader.c fix: param.Name still returns ""
• With both fixes: param.Name correctly returns null

Why

ParameterInfo.Name is nullable (string?) per API Contract)

Related Issues

Fixes failures in dotnet/aspnetcore test suite when running on Mono runtime.

cc: @giritrivedi