FIX: ShuffledDStream run tasks only when dstream has partition items

 ShuffledDStream run tasks no matter whether dstream has partition items 

this patch fixed it
I agree with @jerryshao , if numPartitioner is set to 0, then partitioning will fail as partitions are calculated by doing (hashcode % numPartitions) and anything % 0  gives "java.lang.ArithmeticException: / by zero" error.