IMPALA-1380: EmptySetNode with no tuples generates invalidate thrift messages

TestLastDdlTimeUpdate fails with a query that generates an EmptySetNode and
no tuple ids because the thrift PlanNode requires the row_tuples and
nullable_tuples lists to be set, even if they're empty.

Change-Id: I7177d822e4901cdf200eee50414591c7d73c172c
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4823
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins