IMPALA-1370: Fix NULL literal types in HS2 API

Change-Id: Id5dd2f42ca167726351b220193704770512e94cc
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5017
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins
(cherry picked from commit 37a74ec8e6e0c3f2e67db7233a7ffae52881d231)
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5021