Before:

After:

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>