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

![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.
(cherry picked from commit 96d31fe0a8b88c09488989cd5459d4124dcb7983)

models/actions/schedule_list.go

@@ -44,6 +44,9 @@ func (schedules ScheduleList) LoadTriggerUser(ctx context.Context) error {
 			schedule.TriggerUser = user_model.NewActionsUser()
 		} else {
 			schedule.TriggerUser = users[schedule.TriggerUserID]
+			if schedule.TriggerUser == nil {
+				schedule.TriggerUser = user_model.NewGhostUser()
+			}
 		}
 	}
 	return nil

services/actions/notifier_helper.go

@@ -560,12 +560,9 @@ func DetectAndHandleSchedules(ctx context.Context, repo *repo_model.Repository)
 	}
 
 	// We need a notifyInput to call handleSchedules
-	// Here we use the commit author as the Doer of the notifyInput
-	commitUser, err := user_model.GetUserByEmail(ctx, commit.Author.Email)
-	if err != nil {
-		return fmt.Errorf("get user by email: %w", err)
-	}
-	notifyInput := newNotifyInput(repo, commitUser, webhook_module.HookEventSchedule)
+	// if repo is a mirror, commit author maybe an external user,
+	// so we use action user as the Doer of the notifyInput
+	notifyInput := newNotifyInput(repo, user_model.NewActionsUser(), webhook_module.HookEventSchedule)
 
 	return handleSchedules(ctx, scheduleWorkflows, commit, notifyInput, repo.DefaultBranch)
 }