=act #13678 #15149 Reply with ActorIdentity(None) from deadLetters

(cherry picked from commit b2f96668baf9efa77de5f97223f055c0a78d0cb8)
Merge pull request #15348 from akka/wip-13678-identify-deadLetters-master-patriknw

=act #13678 #15149 Reply with ActorIdentity(None) from deadLetters (for verification)