add ActorRef.noSender() for the Java API, see #3429

- Actor.noSender is not accessible from Java, but it was in 2.1 so don’t
  remove
- replaced all “null” in doc tests with ActorRef.noSender()

akka-docs/rst/java/code/docs/event/LoggingDocTest.java

@@ -36,7 +36,7 @@ public class LoggingDocTest {
   public void useLoggingActor() {
     ActorSystem system = ActorSystem.create("MySystem");
     ActorRef myActor = system.actorOf(Props.create(MyActor.class, this));
-    myActor.tell("test", null);
+    myActor.tell("test", ActorRef.noSender());
     JavaTestKit.shutdownActorSystem(system);
   }