Complain about missing merge-base in EE specific check

We should bail out whenever EE specific check cannot find the merge-base, instead of silently allowing it to continue and accidentally make it pass. (or not)

I think I saw this a few times already. Some reference: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5532#note_82858960

Quoting here:

Looking at ee-files-location-check, specifically this line:

=> Running `git diff ...HEAD --name-status --diff-filter=A -- ./ ':!ee' | cut -f2`

We could see that ...HEAD, and that means ee_fetch_base was nil, meaning that this doesn't give us any result:

=> Running `git merge-base canonical-ee/master HEAD`

I hit into something like this in the past. It usually meant we didn't fetch deep enough (or didn't fetch at all).

Looking at this merge request, it's not so far behind EE master. I don't know why we can't find the EE merge base against HEAD.