=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.
Merge pull request #1645 from akka/wip-3517-AdaptiveLoadBalancingRouter-off-by-one-patriknw

=clu #3517 Guard against wrong random selection