Update TupleIsNullPredicate to not crash in case of bad plans.

Change-Id: Ia1500d7f62fe28b257c2dbbd2f0e2bfcd43b2c24
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5430
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins