=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.

akka-actor-tests/src/test/scala/akka/util/ByteStringSpec.scala

@@ -231,7 +231,7 @@ class ByteStringSpec extends WordSpec with MustMatchers with Checkers {
     for (i ← 0 until data.length) builder.putLongPart(data(i), nBytes)(byteOrder)
 
     reference.zipWithIndex.collect({ // Since there is no partial put on LongBuffer, we need to collect only the interesting bytes
-      case (r, i) if byteOrder == ByteOrder.LITTLE_ENDIAN && i % elemSize < nBytes     ⇒ r
+      case (r, i) if byteOrder == ByteOrder.LITTLE_ENDIAN && i % elemSize < nBytes            ⇒ r
       case (r, i) if byteOrder == ByteOrder.BIG_ENDIAN && i % elemSize >= (elemSize - nBytes) ⇒ r
     }).toSeq == builder.result
   }