Add null check for database in HS2 MetadataOp

It is possible the database was dropped between the call to getDbNames() and
getDb(dbName) causing a NPE.

Change-Id: Icc4d0f5c54995783dd59d73f2db3050369a6f944
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4343
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins