Add partition events may contain empty partition object list

When event polling is ON and when an external application like Hive issues a alter table <table> add if not exists partition (<partspec>) it is possble that command did not add a partition since it is preexisting. However, metastore still generates a ADD_PARTITION event in such a case with empty list of added partitions. Such events cause a Precondition to fail while processing on the EventsProcessor side and event polling goes into error state.

The fix would be simple. Ignore such events.