Backport #30357 by @yp05327
![image](https://github.com/go-gitea/gitea/assets/18380374/ddf6ee84-2242-49b9-b066-bd8429ba4d76)
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)