Backport #30357 by @yp05327
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When repo is a mirror, and commit author is an external user, then
`GetUserByEmail` will return error.
reproduce/test:
- mirror Gitea to your instance
- disable action and enable it again, this will trigger
`DetectAndHandleSchedules`
ps: also follow #24706, it only fixed normal runs, not scheduled runs.
Co-authored-by: yp05327 <576951401@qq.com>
(cherry picked from commit 69cc79173ddbf7662c4d7246b6161b9351038d16)