hailtop.batch errs when a batch of zero jobs is submitted and waited on

[danking]

What happened?

Reported by Wenhan: https://hail.zulipchat.com/#narrow/stream/223457-Hail-Batch-support/topic/ValueError.3A.20min.28.29.20arg.20is.20an.20empty.20sequence/near/352319161

  File "/usr/local/Caskroom/miniconda/base/lib/python3.9/site-packages/hailtop/batch/backend.py", line 791, in _async_run
    starting_job_id = min(j._client_job.job_id for j in unsubmitted_jobs)

I suspect we do not handle waiting on an empty batch / batch update correctly. We should probably bail out immediately from the wait when there are no jobs on which to wait.

Version

batch

Relevant log output

No response

[daniel-goldstein]

Looks like we emit a warning here, but ya we should probably bail out before we get to this line because starting_job_id makes no sense if there are no jobs.