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/actor/InboxDocTest.java

@@ -49,7 +49,7 @@ public class InboxDocTest {
     //#watch
     final Inbox inbox = Inbox.create(system);
     inbox.watch(target);
-    target.tell(PoisonPill.getInstance(), null);
+    target.tell(PoisonPill.getInstance(), ActorRef.noSender());
     assert inbox.receive(Duration.create(1, TimeUnit.SECONDS)) instanceof Terminated;
     //#watch
   }