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/camel/OnRouteResponseTestBase.java

@@ -14,7 +14,7 @@ public class OnRouteResponseTestBase {
         Forwarder.class, "http://localhost:8080/news/akka", receiver));
     // the Forwarder sends out a request to the web page and forwards the response to
     // the ResponseReceiver
-    forwardResponse.tell("some request", null);
+    forwardResponse.tell("some request", ActorRef.noSender());
     //#RouteResponse
     system.stop(receiver);
     system.stop(forwardResponse);