Fix ispconfig/ispconfig3#6380: Mailusers cannot login.

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']) and $user['otp_type'] != 'none') {
 
 				//* Save session in pending state and destroy original session
 				$_SESSION['s_pending'] = $_SESSION['s'];