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/jrouting/RouterViaProgramDocTest.java

@@ -55,7 +55,7 @@ public class RouterViaProgramDocTest {
         Props.empty().withRouter(new RoundRobinRouter(routees)));
       //#programmaticRoutingRouteePaths
       for (int i = 1; i <= 6; i++) {
-        router.tell(new ExampleActor.Message(i), null);
+        router.tell(new ExampleActor.Message(i), ActorRef.noSender());
       }
     }};
   }