Fix typehint of $config argument in Purifier::clean

Author: gisostallenberg

src/Purifier.php

@@ -196,7 +196,7 @@ private function checkCacheDirectory()
     }
 
     /**
-     * @param null $config
+     * @param array<string, mixed>|string|null $config
      *
      * @return mixed|null
      */
@@ -256,7 +256,7 @@ protected function getConfig($config = null)
 
     /**
      * @param      $dirty
-     * @param null $config
+     * @param array<string, mixed>|string|null $config
      * @param \Closure|null $postCreateConfigHook
      * @return mixed
      */