Fix CI runner validation

What does this MR do and why?

In Rails 7, #one? executes a COUNT query even though there are unpersisted records in the association. This may be a bug in Rails 7 but I haven't investigated further.

In this MR, I just change the validation to use #size since they are similar. This also executes a COUNT query when the associated records are persisted and the relation is not loaded yet.

This is extracted from !90907 (merged) to make the MR smaller

Example Rails 7 failure can be seen in https://gitlab.com/gitlab-org/gitlab/-/jobs/3946108357

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