IMPALA-1524: Materialize all tuples produced by an EmptySetNode.

Change-Id: I3b151ace464c67634104f84f7223c948fed8909e
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5406
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins