PIG-3568: Define semantics of STATUS_NULL
Review Request #15524 - Created Nov. 14, 2013 and updated
This patch defines STATUS_NULL to mean that no result was produced and changes all conflicts to conform with this interpretation.
'ant test-commit' on this patch, and 'ant test' before merging PIG-3570. There were a couple failures from 'ant test', but they're unrelated