[mono][System.Reflection.Emit] Fix explicitly setting the return type on a dynamic method

[ivanpovazan]

Description

This PR fixes the support of SetReturnType and SetParameters in the MethodBuilder API with Mono, by properly setting the return type of the dynamic method only if it is non null value.

This was manifested in two scenarios:

1. If the dynamic method return and parameter types are manually set in the following sequence:

    methodBuilder.SetReturnType(interfaceMethodInfo.ReturnType);
    methodBuilder.SetParameters(paramTypes);

   The subsequent call to SetParameters (which internally calls SetSignatureCore) would overwrite the set return type to null causing the method not to be considered as an implementation of a interface method. This scenario is used in https://github.com/dotnet/aspnetcore/blob/4dc81f80abd45cb5c2e04d3b742bc6755c1153ee/src/SignalR/server/Core/src/Internal/TypedClientBuilder.cs#L136-L137 causing ASP.NET test failures when using Mono.

2. Setting explicit null as a return type via SetReturnType overwrites the default return type value which should be typeof(void)

   This scenario was covered with System.Reflection.Emit libraries tests

   runtime/src/libraries/System.Reflection.Emit/tests/MethodBuilder/MethodBuilderSetReturnType.cs

   Lines 141 to 143 in 4fa760f

   	method.SetReturnType(null);
   	type.CreateType();
   	VerifyReturnType(method, typeof(void));

   but was disabled for Mono as reported in: [mono] Tests failed: System.Reflection.Emit.Tests #2389

Verification

This fix was verified by enabling the disabled unit test (and verifying locally that it passes) and manually verifying that the repro-iclientproxy-vtable-fail as described in #94490 (comment) does not crash.

---

Fixes #94490

[ivanpovazan]

Once this fix gets approved, we should consider backporting it to .net8 servicing release as it affects signalR users on RHEL.

[buyaa-n]

Thank you for the fix.

Once this fix gets approved, we should consider backporting it to .net8 servicing release as it affects signalR users on RHEL.

💯, this looks regression introduced in 8.0 with #78544

[ivanpovazan]

/backport to release/8.0-staging

[github-actions]

Started backporting to release/8.0-staging: https://github.com/dotnet/runtime/actions/runs/7007281730