FIX: ShuffledDStream run tasks only when dstream has partition items

 ShuffledDStream run tasks no matter whether dstream has partition items 

this patch fixed it
Hi @guowei2 , two quick questions:
1. Is there any situation where partitions will be 0?
2. I don't think `numPartitions` can be set to 0, since HashPartitioner will be failed if `Mod 0`.