Fix 500 error when select `No assignee` filter in issue list page (#24854)

Fix 500 error when select `No assignee` filter in issue list page (#24854)

Access the issue list page and select `No assignee`, you will get 500
error:

![image](https://github.com/go-gitea/gitea/assets/18380374/a7435fd3-c3d5-4801-894e-9353a8ddc1af)

You can see this page in try.gitea.io:

https://try.gitea.io/yp05327/testrepo/issues?q=&type=all&sort=&state=open&labels=&milestone=0&project=0&assignee=-1&poster=0

Caused by
https://github.com/go-gitea/gitea/pull/24707/files#diff-d7c7dc2da4fcfdc09997648e11a018414dc116472054350b5590ab407adcd44dR1748

Co-authored-by: Giteabot <teabot@gitea.io>