PIG-2828: Handle nulls in DataType.compare

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1489223 13f79535-47bb-0310-9956-ffa450edef68
PIG-3322: AvroStorage give NPE on reading file with union as top level schema (viraj via rohini)

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1489264 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk