New tests for atomic ops

Src/Myc.Core.Signals.pas

@@ -212,6 +212,7 @@ function TMycLatch.Subscribe(Subscriber: IMycSubscriber): Pointer;
 var
     alreadySet: Boolean;
 begin
+    Result := nil;
     if not Assigned(Subscriber) then
         exit;
 
@@ -332,7 +333,7 @@ end;
 function TMycDirty.Subscribe(Subscriber: IMycSubscriber): Pointer;
 begin
     if not Assigned(Subscriber) then
-        exit;
+        exit(nil);
 
     // For TMycDirty, we always add the subscriber and then check if we need to notify immediately.
     // Ref counting for the subscriber interface is assumed to be handled by TMycNotifyList.Advise/Unadvise.