Fixed warning 'passing NULL to non-pointer arguement' in expr.cc

Change-Id: I836c873281cf415b2d952a4b46d6eb3ac5a12bdd
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3523
Reviewed-by: Paden Tomasello <paden.tomasello@cloudera.com>
Tested-by: jenkins