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()
Merge pull request #1529 from akka/wip-3429-ActorRef.noSender-∂π

add ActorRef.noSender() for the Java API, see #3429