Fix refreshing of issues/MR count caches

What does this MR do?

This fixes the updating of the open issues/MRs count caches. This was broken but would work in the test suite due to the use of a null cache. This is a fix for a change introduced in 10.0, so no changelog has been added.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/38061