HIVE-7745 - NullPointerException when turn on hive.optimize.union.remove, hive.merge.mapfiles and hive.merge.mapredfiles [Spark Branch] (Na Yang via Brock)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/spark@1618708 13f79535-47bb-0310-9956-ffa450edef68