Resolve "ee-specific-check checked against the wrong base when there's no CE branch"

What does this MR do?

Just pick the CE merge base when there's no CE branch

Are there points in the code the reviewer needs to double check?

I put git_run_command("merge-base #{ce_merge_base} HEAD") before in case the EE branch was behind the CE merge-base. When that happened, I think something would be wrong. However, I just tried to reproduce the issue I hit before, and it looks like I can't really reproduce it now, and just using the CE merge base is working properly.

Well, I think even this is not completely solving the issue, it's at least better than the current status.

Let's see...

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

Closes #6890 (closed)