refactor: remove unneeded isset($config['except']

To fix PHPStan error:
Error: Offset 'except' on array in isset() always exists and is not nullable.
 ------ --------------------------------------------------------------
  Line   src/Auth.php
 ------ --------------------------------------------------------------
  109    Offset 'except' on array in isset() always exists and is not
         nullable.
 ------ --------------------------------------------------------------

Author: kenjis

src/Auth.php

@@ -106,7 +106,7 @@ public function routes(RouteCollection &$routes, array $config = []): void
 
         $routes->group('/', ['namespace' => 'CodeIgniter\Shield\Controllers'], static function (RouteCollection $routes) use ($authRoutes, $config): void {
             foreach ($authRoutes as $name => $row) {
-                if (! isset($config['except']) || (isset($config['except']) && ! in_array($name, $config['except'], true))) {
+                if (! isset($config['except']) || ! in_array($name, $config['except'], true)) {
                     foreach ($row as $params) {
                         $options = isset($params[3])
                             ? ['as' => $params[3]]