HIVE-6704 : date_add()/date_sub()/datediff() fail with NPE with null input (Jason Dere via Ashutosh Chauhan)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/branch-0.13@1580539 13f79535-47bb-0310-9956-ffa450edef68