fix(api): keep milestone due date when it is omitted in edit request (#39869)

`PATCH /repos/{owner}/{repo}/milestones/{id}` always overwrote the due
date with `due_on`, so a request that only changed other fields, such as
`{"state": "closed"}`, silently removed it.

The due date is now kept when `due_on` is omitted, and `"due_on": null`
or `"due_on": ""` remove it.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/issues/issue_update.go

@@ -529,7 +529,7 @@ func UpdateIssueMentions(ctx context.Context, issueID int64, mentions []*user_mo
 
 // UpdateIssueDeadline updates an issue deadline and adds comments. Setting a deadline to 0 means deleting it.
 func UpdateIssueDeadline(ctx context.Context, issue *Issue, deadlineUnix timeutil.TimeStamp, doer *user_model.User) (err error) {
-	// if the deadline hasn't changed do nothing
+	// if the deadline hasn't changed, do nothing
 	if issue.DeadlineUnix == deadlineUnix {
 		return nil
 	}
@@ -539,11 +539,11 @@ func UpdateIssueDeadline(ctx context.Context, issue *Issue, deadlineUnix timeuti
 		if err = UpdateIssueCols(ctx, &Issue{ID: issue.ID, DeadlineUnix: deadlineUnix}, "deadline_unix"); err != nil {
 			return err
 		}
-
 		// Make the comment
 		if _, err = createDeadlineComment(ctx, doer, issue, deadlineUnix); err != nil {
-			return fmt.Errorf("createRemovedDueDateComment: %w", err)
+			return fmt.Errorf("createDeadlineComment: %w", err)
 		}
+		issue.DeadlineUnix = deadlineUnix
 		return nil
 	})
 }