Merge branch '6380-mailuser-otp' into 'develop'

Fix mailuser login, closes #6380

Closes #6380

See merge request ispconfig/ispconfig3!1627

Author: Till Brehm

interface/web/login/index.php

@@ -140,7 +140,7 @@ function process_login_request(app $app, &$error, $conf, $module)
 		} else {
 
 			//* Do 2FA authentication
-			if($user['otp_type'] != 'none') {
+			if(isset($user['otp_type']) && $user['otp_type'] != 'none') {
 
 				//* Save session in pending state and destroy original session
 				$_SESSION['s_pending'] = $_SESSION['s'];