Fix nyxt scheme callback logic

[aadcg]

Description

Checklist:

• [x] Git branch state is mergable.
• [x] Changelog is up to date (via a separate commit).
• [x] New dependencies are accounted for.
• [x] Documentation is up to date.
• [x] Compilation and tests ((asdf:test-system :nyxt/<renderer>))
  • No new compilation warnings.
  • Tests are sufficient.