Avoid panic caused by broken payload when creating commit status (#23216)

When creating commit status for Actons jobs, a payload with nil
`HeadCommit` will cause panic.

Reported at:
https://gitea.com/gitea/act_runner/issues/28#issuecomment-732166

Although the `HeadCommit` probably can not be nil after #23215,
`CreateCommitStatus` should protect itself, to avoid being broken in the
future.

In addition, it's enough to print error log instead of returning err
when `CreateCommitStatus` failed.

---------

Co-authored-by: delvh <dev.lh@web.de>

routers/api/actions/runner/runner.go

@@ -150,7 +150,7 @@ func (s *Service) UpdateTask(
 	}
 
 	if err := actions_service.CreateCommitStatus(ctx, task.Job); err != nil {
-		log.Error("Update commit status failed: %v", err)
+		log.Error("Update commit status for job %v failed: %v", task.Job.ID, err)
 		// go on
 	}