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/SchedulerDocTest.java

@@ -41,7 +41,7 @@ public class SchedulerDocTest {
       new Runnable() {
         @Override
         public void run() {
-          testActor.tell(System.currentTimeMillis(), null);
+          testActor.tell(System.currentTimeMillis(), ActorRef.noSender());
         }
     }, system.dispatcher());
     //#schedule-one-off-thunk