Before:
![image](https://github.com/go-gitea/gitea/assets/18380374/1ab476dc-2f9b-4c85-9e87-105fc73af1ee)
After:
![image](https://github.com/go-gitea/gitea/assets/18380374/786f984d-5c27-4eff-b3d9-159f68034ce4)
This issue comes from the change in #25468.
`LoadProject` will always return at least one record, so we use
`ProjectID` to check whether an issue is linked to a project in the old
code.
As other `issue.LoadXXX` functions, we need to check the return value
from `xorm.Session.Get`.
In recent unit tests, we only test `issueList.LoadAttributes()` but
don't test `issue.LoadAttributes()`. So I added a new test for
`issue.LoadAttributes()` in this PR.
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Co-authored-by: Denys Konovalov <privat@denyskon.de>