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>

services/actions/notifier_helper.go

@@ -187,7 +187,8 @@ func notify(ctx context.Context, input *notifyInput) error {
 		} else {
 			for _, job := range jobs {
 				if err := CreateCommitStatus(ctx, job); err != nil {
-					log.Error("CreateCommitStatus: %v", err)
+					log.Error("Update commit status for job %v failed: %v", job.ID, err)
+					// go on
 				}
 			}
 		}