Properly handle and return empty string for dangling commits in GetBranchName (#11587)

models/issue_comment.go

@@ -1129,7 +1129,7 @@ func getCommitIDsFromRepo(repo *Repository, oldCommitID, newCommitID, baseBranch
 		return nil, false, err
 	}
 
-	if oldCommitBranch == "undefined" {
+	if oldCommitBranch == "" {
 		commitIDs = make([]string, 2)
 		commitIDs[0] = oldCommitID
 		commitIDs[1] = newCommitID