allow null response to Ask<T> operation

src/core/Akka.Tests/Actor/AskSpec.cs

@@ -8,10 +8,12 @@
 using Akka.TestKit;
 using Xunit;
 using Akka.Actor;
+using Akka.Actor.Dsl;
 using System;
 using System.Threading;
 using System.Threading.Tasks;
 using Akka.Util.Internal;
+using FluentAssertions;
 using Nito.AsyncEx;
 
 namespace Akka.Tests.Actor
@@ -180,6 +182,22 @@ public async Task ShouldFailWhenAskExpectsWrongType()
             // expect int, but in fact string
             await Assert.ThrowsAsync<ArgumentException>(async () => await actor.Ask<int>("answer"));
         }
+
+        /// <summary>
+        /// Reproduction for https://github.com/akkadotnet/akka.net/issues/5204
+        /// </summary>
+        [Fact]
+        public async Task Bugfix5204_should_allow_null_response_without_error()
+        {
+            var actor = Sys.ActorOf(act => act.ReceiveAny((o, context) =>
+            {
+                context.Sender.Tell(null);
+            }));
+
+            // expect a string, but the answer should be `null`
+            var resp = await actor.Ask<string>(1);
+            resp.Should().BeNullOrEmpty();
+        }
 
         [Fact]
         public void AskDoesNotDeadlockWhenWaitForResultInGuiApplication()

src/core/Akka/Actor/ActorRef.cs

@@ -134,6 +134,10 @@ protected override void TellInternal(object message, IActorRef sender)
                     {
                         _result.TrySetResult(t);
                     }
+                    else if (message == null) //special case: https://github.com/akkadotnet/akka.net/issues/5204
+                    {
+                        _result.TrySetResult(default);
+                    }
                     else if (message is Failure f)
                     {
                         _result.TrySetException(f.Exception ?? new TaskCanceledException("Task cancelled by actor via Failure message."));