Switch Zero's returned schema to tuple

When using current svn's pig, zero_carrier.pig failed with

    <file zero_carrier.pig, line 37, column 14> pig script failed to validate: org.apache.pig.impl.logicalLayer.FrontendException: ERROR 0: Given UDF returns an improper Schema. Schema should only contain one field of a Tuple, Bag, or a single type. Returns: {chararray,chararray}

. We fix the output schema of Zero to return a tuple.

Change-Id: I09f18af429bd655b673072e37336f02d98811ba2
Merge "Switch Zero's returned schema to tuple"