CDH-20089: Query planning failed in HdfsScanNode.evalBinaryPredicate

This commit fixes issue CDH-20089 where an error is thrown when we have
a binary predicate on a partition key that has no values.

Change-Id: I3b5cefb4d7193045fc6fc5e94766589c2299b5b1
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3327
Reviewed-by: Dimitris Tsirogiannis <dtsirogiannis@cloudera.com>
Tested-by: jenkins