fix `SHARD_BATCH_SIZE=0` support

[wwared]

SHARD_BATCH_SIZE=0 does not work the same way as it used to anymore after #40, and if you set it will panic with "chunk size must be non-zero" here

As far as I can tell this is also an issue in current upstream dev, but it might be possible to change that particular code to handle SHARD_BATCH_SIZE=0 and revert to previous behavior (I think), hopefully in a minimally intrusive way

This is because this check still exists, but no longer works due to the new code here

Originally posted by @wwared in #40 (comment)

[wwared]

Fixed by #40