Bug fixes in null-aware anti-join

This commit fixes issue IMPALA-1215 where NOT IN subqueries return wrong
results in the presence of null values.

Change-Id: I97e41c8df8ba864d0189595d670b3f0349fcad36
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4467
Reviewed-by: Dimitris Tsirogiannis <dtsirogiannis@cloudera.com>
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: Nong Li <nong@cloudera.com>