A fix for a bug introduced by me earlier, where attempting to parse an issue reference in an empty token would crash.
An empty token occurs if the search string is `\` or `"` (among other scenarios, probably). I'll make another PR that avoids having empty tokens (seems like a good idea).
### Tests
- I added test coverage for Go changes...
- [x] in their respective `*_test.go` for unit tests.
### Documentation
- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.
### Release notes
- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8260
Reviewed-by: Shiny Nematoda <snematoda@noreply.codeberg.org>
Co-authored-by: Danko Aleksejevs <danko@very.lv>
Co-committed-by: Danko Aleksejevs <danko@very.lv>
