=clu #3517 Guard against wrong random selection

* When all weights in the map are zero total became 0, which is not
  valid in random pick. Treat this special case as empty WeightedRoutees, i.e.
  no availble targets and routing to deadLetters.
(cherry picked from commit 0afe653fe1db3b4d62e91010f880d5dce0179f14)
Merge pull request #1657 from akka/wip-3517-AdaptiveLoadBalancingRouter-off-by-one-2.2-patriknw

=clu #3517 Guard against wrong random selection (backport)