health: comment job fails when every check is skipped by label #456

Skipping every enabled check with a skip-*-check label makes the comment job fail, so the label trades one red X for another.

A repo that enables a single check:

    uses: dart-lang/ecosystem/.github/workflows/health.yaml@main
    with:
      checks: "changelog"
      fail_on: "changelog"

and labels the PR skip-changelog-check gets a passing changelog job and a failing comment job:

Found 0 artifact(s)
Total of 0 artifact(s) downloaded
Download artifact has finished successfully
##[group]Run ls -R single-comments
ls: cannot access 'single-comments': No such file or directory
##[error]Process completed with exit code 2.

A skipped check never reaches writeInComment, so it uploads no artifact. With no artifacts at all, actions/download-artifact creates no directory, and ls -R single-comments exits 2 under bash -e. find single-comments ... in the merge step would fail the same way.

Repos enabling several checks mostly don't hit this, since one unskipped check is enough to produce an artifact. It reliably bites any repo that runs a single check and uses the documented label for it.

mkdir -p single-comments before the ls, or skipping the job when no artifacts were downloaded, would cover it. Related to but distinct from #371, which was about the layout of a non-empty directory.

Example run: https://github.com/a2ui-project/a2ui/actions/runs/33995556417/job/101385428552