This job errors out when only new markdown files are added: https://github.com/mdn/content/actions/runs/22491270321/job/65153850227?pr=43109

Reproduced in this PR with this commit a2be0c8 and this workflow run: https://github.com/mdn/content/actions/runs/22496878636/job/65173642704?pr=43282

And fixed with this commit 255e861 and this workflow run: https://github.com/mdn/content/actions/runs/22496961201/job/65173929031?pr=43282

(The pull_request_target runs of this workflow failed on both commits, as it runs the workflow from main, which still has the bug.)

Also fixes a mixup between HEAD/BASE in a comment and github commit message, and ensures the pull_request target runs of the workflow actually behave in the same way as the pull_request_target ones do.