[mono][System.Reflection.Emit] Fix explicitly setting the return type on a dynamic method (#95271)

Co-authored-by: Ivan Povazan <[email protected]>

Author: github-actions[bot]

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

@@ -132,7 +132,6 @@ public void SetReturnType_TypeCreated_Works()
         }
 
         [Fact]
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/2389", TestRuntimes.Mono)]
         public void SetReturnType_NullReturnType_ReturnsVoid()
         {
             TypeBuilder type = Helpers.DynamicType(TypeAttributes.Abstract);

src/mono/System.Private.CoreLib/src/System/Reflection/Emit/RuntimeMethodBuilder.Mono.cs

@@ -603,7 +603,9 @@ protected override void SetSignatureCore(Type? returnType, Type[]? returnTypeReq
                 Array.Copy(parameterTypes, this.parameters, parameterTypes.Length);
             }
 
-            rtype = returnType;
+            if (returnType != null)
+                rtype = returnType;
+
             returnModReq = returnTypeRequiredCustomModifiers;
             returnModOpt = returnTypeOptionalCustomModifiers;
             paramModReq = parameterTypeRequiredCustomModifiers;