fix: explicit nullable type for PHP 8.4+ compatibility

- Update config() method parameters to use explicit nullable types
- Change `string $key = null` to `?string $key = null`
- Change `$default = null` to `mixed $default = null`
- Fixes deprecation warning in PHP 8.4+

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Tinywan

src/Model/LaravelRuleModel.php

@@ -55,11 +55,11 @@ public function __construct(array $data = [], ?string $driver = null)
      * Gets config value by key.
      *
      * @param string|null $key
-     * @param null $default
+     * @param mixed $default
      *
      * @return mixed
      */
-    protected function config(?string $key = null, $default = null)
+    protected function config(?string $key = null, mixed $default = null)
     {
         $driver = $this->driver ?? config('plugin.casbin.webman-permission.permission.default');
         return config('plugin.casbin.webman-permission.permission.' . $driver . '.' . $key, $default);

src/Model/RuleModel.php

@@ -68,11 +68,11 @@ protected function init(): void
      * Gets config value by key.
      *
      * @param string|null $key
-     * @param null $default
+     * @param mixed $default
      *
      * @return mixed
      */
-    protected function config(string $key = null, $default = null)
+    protected function config(?string $key = null, mixed $default = null)
     {
         $driver = $this->driver ?? config('plugin.casbin.webman-permission.permission.default');
         return config('plugin.casbin.webman-permission.permission.' . $driver . '.' . $key, $default);