Venue deployment: check if group content is empty

[celestemartinez]

If there is an error in the deployment of a venue, the venue group might be created but the group content can be empty. If this happens, we will get an error when re-deploying because we expect the group content to be nonempty when the venue group exists. We should add a check here: https://github.com/openreview/openreview-py/blob/c13b9abb16f8bf015d72d685920b50a4eab9d2b0/openreview/conference/helpers.py#L14C11-L14C11 to see if the group content exists