Send registration email on user autoregistration (#16523)

When users login and are autoregistered send email notification.

Fix #16178

* Protect public functions within the mailer by testing if the mailer is configured

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: 6543 <6543@obermui.de>

services/mailer/mail_repo.go

@@ -9,12 +9,18 @@ import (
 	"fmt"
 
 	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/translation"
 )
 
 // SendRepoTransferNotifyMail triggers a notification e-mail when a pending repository transfer was created
 func SendRepoTransferNotifyMail(doer, newOwner *models.User, repo *models.Repository) error {
+	if setting.MailService == nil {
+		// No mail service configured
+		return nil
+	}
+
 	if newOwner.IsOrganization() {
 		users, err := models.GetUsersWhoCanCreateOrgRepo(newOwner.ID)
 		if err != nil {