gh-37147: fix doctest in gap_packages (using empty table)
    
This is fixing a misbehaviour of empty tables, in their new unicode
clothes.

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have created tests covering the changes.
    
URL: #37147
Reported by: Frédéric Chapoton
Reviewer(s): Travis Scrimshaw

Author: Release Manager