Use Map.empty for empty receive.

{ case _ => } siliently ignores any message given to this function. This makes
debugging hard. Use Map.empty instead so that
akka.actor.UnhandledMessage(message, sender, recipient) will be published to
the ActorSystem's EventStream.
Turn akka.actor.debug.unhandled on only in dev mode.

This option converts UnhandledMessages into actual Debug messages. This is
useful in dev mode, but too verbose in other modes.