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/remoting/RemoteDeploymentDocTest.java

@@ -53,7 +53,7 @@ public class RemoteDeploymentDocTest {
     //#sample-actor
 
     ActorRef actor = system.actorOf(Props.create(SampleActor.class), "sampleActor");
-    actor.tell("Pretty slick", null);
+    actor.tell("Pretty slick", ActorRef.noSender());
     //#sample-actor
   }