Enqueueing creates an empty student group

In course edition 370, there were some issues with student groups. There was a single student group that all requests were assigned to. Upon investigation, it seems that this student group is an empty student group with the name of a single student, causing more issues.

The issue seems mostly related to the added pieces of code for !597 (merged). It might be that a solution is to double check that a created group in fact has a single member, and to check that a member added to an empty group actually is added to that group.