IMPALA-1535: Partition pruning with NULL

This commit fixes the issue where partition pruning returns wrong
results when a binary predicate contains a NULL literal.

Change-Id: I24c647184dcef49d12d6ff422e28667777df7784
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5443
Reviewed-by: Dimitris Tsirogiannis <dtsirogiannis@cloudera.com>
Tested-by: jenkins