NPE thrown when reading legacy ACID delta files

When reading legacy ACID deltas of the form delta_$startTxnId_$endTxnId a NullPointerException is thrown on:

if(dmd.getStmtIds().isEmpty()) {

The older ACID data format (pre-Hive 1.3.0) which does not include the statement ID, and code written for that format should still be supported. Therefore the above condition should also include a null check or alternatively AcidInputFormat.DeltaMetaData should never return null, and return an empty list in this specific scenario.