[CDH4] Fix inverted NULLs for HS2 metadata ops

This patch fixes an bug where, for HS2 metadata ops, all values were
marked as null (unless they were null, in which case they were marked as
not null...). This didn't show up because the CDH4 JDBC driver doesn't
speak HS2 V6, which is the only coverage we had for metadata operations
in HS2.

TODO: tests/hs2/ should have metadata tests.

Change-Id: Id45a99b35b7328d171f4c3b3639495fc502bfc83
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4411
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins