serverless: clean up handling tenants with old naming schema

As Steven pointed out in [this](https://github.com/cockroachdb/cockroach/pull/114213#discussion_r1389744218) PR review comment:
> I think we may still have some tenants with no name field. We've made such tenants very hard to produce in 23.1. I do think this points out another potential behaviour difference because of this change. Namely, in CC I think some people will get VC names in their shell and others won't depending on when their tenant was created. It's potentially also true that some people will see `tenant-%d` (the old naming scheme) while some will see `cluster-%d` the new naming scheme. Maybe we can fix this by renaming a bunch of tenants in serverless.

We should check whether we need to do something here or not.

Jira issue: CRDB-33424