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
(cherry picked from commit eaaeb41273fff5811758ea82cad0a60a9b6d52b0)
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5457
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>