Avoid user does not exist error when detecting schedule actions when the commit author is an external user (#30357)

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.
(cherry picked from commit 96d31fe0a8b88c09488989cd5459d4124dcb7983)
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