HIVE-6753: Unions on Tez NPE when there's a mapjoin the union work (Gunther Hagleitner, reviewed by Vikram Dixit K)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/branch-0.13@1582501 13f79535-47bb-0310-9956-ffa450edef68