Contributors script sometimes adds @unknown to the thank-you section.

The contributors script adds the user called unknown to the thank-you section when it cannot retrieve the actual username of a contributor.


I ran:

```terminal
python /home/lengau/Work/Code/starflow/tools/contributors.py --project=charmcraft --old-ref=4.1.0 --new-ref=3b6de4ae53dd8342f4f82797017882728b7763a2
```