TezIndexRecord#hasData() returns true for empty index record in the Unordered case

In the case of Unordered writer, use of hasData() returns true for empty partitions since the rawlength is zero (as writer skips writing them out altogether) and the check in hasData will fail as rawlength == header.length +2 fails.

This currently works for the ordered case(and the method is used here unlike in unordered writer) but after TEZ-3605, it will fail for both scenarios. Even if we don't use the method, the logic should be fixed.