[Patch] Fix closed PR also triggers Webhooks and actions (#23782) (#23834)

Backport #23782 by @sillyguodong

Fix #23707 
Cause by #23189
This PR is a quick fix that, when pushing commits to closed PR, webhook
and actions also be triggered.

Co-authored-by: sillyguodong <33891828+sillyguodong@users.noreply.github.com>

services/pull/pull.go

@@ -294,6 +294,10 @@ func AddTestPullRequestTask(doer *user_model.User, repoID int64, branch string,
 			}
 			if err == nil {
 				for _, pr := range prs {
+					if pr.Issue.IsClosed {
+						// The closed PR never trigger action or webhook
+						continue
+					}
 					if newCommitID != "" && newCommitID != git.EmptySHA {
 						changed, err := checkIfPRContentChanged(ctx, pr, oldCommitID, newCommitID)
 						if err != nil {