[CI] cSpell GitHub action doesn't always check files in a PR

[chalin]

Originally posted by @chalin in #4956 (comment). The run log is from #4949.

@cartermp - good question. It seems that the GH action didn't find any files to check against (which is clearly a bug), https://github.com/open-telemetry/opentelemetry.io/actions/runs/10251105861/job/28358372032:

Run streetsidesoftware/cspell-action@v6
  with:
    files: content/en/*.md
  layouts/**/*.md
  data/**/*
  
    config: .cspell.yml
    incremental_files_only: true
    inline: warning
    treat_flagged_words_as_errors: false
    strict: true
    suggestions: false
    verbose: false
    check_dot_files: explicit
    use_cspell_files: false
cspell-action
Pull Request
Files checked: 0, Issues found: 0 in 0 files.