remove exception in publish_batch if batch_messages is empty

I think it is completely legitimate that batch_messages be empty.

Looking at c37ffff36ac911d3aa82d96e4b322036ff5642d9, with the current behavior, the example would even throw an exception in the case that the latest iteration match if ($i % $batch == 0). Which might be the case with $batch = 10 and $max = 101 if I'm not mistaken.