PIG-3434: Null subexpression in bincond nullifies outer tuple (or bag)

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1527774 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk
Merge remote branch 'origin/trunk' into trunk-gd