Actions: Use default branch as ref when a branch/tag delete occurs (#23910)

Currently using the tip of main
(2c585d62a4) and when deleting a branch
(and presumably tag, but not tested), no workflows with `on: [delete]`
are being triggered. The runner isn't being notified about them. I see
this in the gitea log:

`2023/04/04 04:29:36 ...s/notifier_helper.go:102:Notify() [E] an error
occurred while executing the NotifyDeleteRef actions method:
gitRepo.GetCommit: object does not exist [id: test, rel_path: ]`

Understandably the ref has already been deleted and so `GetCommit`
fails. Currently at
https://github.com/go-gitea/gitea/blob/main/services/actions/notifier_helper.go#L130,
if the ref is an empty string it falls back to the default branch name.
This PR also checks if it is a `HookEventDelete` and does the same.
Currently `${{ github.ref }}` would be equivalent to the deleted branch
(if `notify()` succeded), but this PR allows `notify()` to proceed and
also aligns it with the GitHub Actions behavior at
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#delete:

`$GITHUB_REF` / `${{ github.ref }}` => Default branch (main/master)
`$GITHUB_SHA` / `${{ github.sha }}` => Last commit on default branch

If the user needs the name of the deleted branch (or tag), it is
available as `${{ github.event.ref }}`.

There appears to be no way for the user to get the tip commit SHA of the
deleted branch (GitHub does not do this either).

N.B. there may be other conditions other than `HookEventDelete` where
the default branch ref needs swapped in, but this was sufficient for my
use case.

services/actions/notifier_helper.go

@@ -127,6 +127,11 @@ func notify(ctx context.Context, input *notifyInput) error {
 	defer gitRepo.Close()
 
 	ref := input.Ref
+	if input.Event == webhook_module.HookEventDelete {
+		// The event is deleting a reference, so it will fail to get the commit for a deleted reference.
+		// Set ref to empty string to fall back to the default branch.
+		ref = ""
+	}
 	if ref == "" {
 		ref = input.Repo.DefaultBranch
 	}