NoSender is changed to null (Fixes for NoSender serialization bugs)

[ssoshchepkov]

Fixes for NoSender actor reference serialization bugs described in #1036.
NoSender actor reference is changed to null as discussed in #1037.

[rogeralsing]

There seems to be some side effect to this.
Possibly just an error in some spec

System.NullReferenceException : Object reference not set to an instance of an object.
   at System.Object.GetType()
   at Akka.TestKit.TestKitBase.InternalExpectMsgEnvelope[T](Nullable`1 timeout, Action`2 assert, String hint, Boolean shouldLog) in d:\BuildAgent\work\49b164d63843fb4\src\core\Akka.TestKit\TestKitBase_Expect.cs:line 202
   at Akka.TestKit.TestKitBase.InternalExpectMsg[T](Nullable`1 timeout, Action`2 assert, String hint) in d:\BuildAgent\work\49b164d63843fb4\src\core\Akka.TestKit\TestKitBase_Expect.cs:line 166
   at Akka.TestKit.TestKitBase.ExpectMsg[T](Predicate`1 isMessage, Nullable`1 timeout, String hint) in d:\BuildAgent\work\49b164d63843fb4\src\core\Akka.TestKit\TestKitBase_Expect.cs:line 57
   at Akka.Testkit.Tests.NoImplicitSenderSpec.When_Not_ImplicitSender_then_testActor_is_not_sender() in d:\BuildAgent\work\49b164d63843fb4\src\core\Akka.TestKit.Tests\NoImplicitSenderSpec.cs:line 22

[ssoshchepkov]

Yes, in this spec actor sends NoSender as a message:

public class NoImplicitSenderSpec : AkkaSpec, INoImplicitSender
    {
        [Fact]
        public void When_Not_ImplicitSender_then_testActor_is_not_sender()
        {
            var echoActor = Sys.ActorOf(c => c.ReceiveAny((m, ctx) => TestActor.Tell(ctx.Sender)));
            echoActor.Tell("message");
            ExpectMsg<IActorRef>(actorRef => Equals(actorRef, ActorRefs.NoSender));
        }

    }

TestActor.Tell(ctx.Sender) sends null and then message.GetType() for this message causes NullReferenceException:

_assertions.Fail(failMessage2, typeof(T), message.GetType(), message, hint ?? "", sender);

[rogeralsing]

Could you mark the failing test with skip attribute?
Just so we dont merge failing tests that confuse others before the test itself is cleaned up or removed

[rogeralsing]

Thanks!