Update TupleIsNullPredicate to not crash in case of bad plans.
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 <99a5e5f8f5911755b88e0b536d46aafa102bed41@cloudera.com>
Tested-by: jenkins

ImpalaToGo/ImpalaToGo,ImpalaToGo/ImpalaToGo,placrosse/ImpalaToGo,placrosse/ImpalaToGo,placrosse/ImpalaToGo,placrosse/ImpalaToGo,ImpalaToGo/ImpalaToGo,ImpalaToGo/ImpalaToGo,ImpalaToGo/ImpalaToGo,placrosse/ImpalaToGo,ImpalaToGo/ImpalaToGo,placrosse/ImpalaToGo