IMPALA-1297: Results of NOT IN may not be correct if subquery results in
NULL

This commit fixes a bug in the implementation of the null-aware anti
join that resulted in wrong results being returned from NOT IN correlated
subqueries in the presence of nulls.

Change-Id: I6f2eb326ec7e40d80ec8da94ba33946b9ac9b115
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4506
Reviewed-by: Dimitris Tsirogiannis <dtsirogiannis@cloudera.com>
Tested-by: jenkins