Update Config.php for PHP 8.4

Ref drush-ops/drush#6069

src/Config.php

@@ -30,7 +30,7 @@ class Config implements ConfigInterface, ConfigInterpolatorInterface
      * @param array $data
      *   Config data to store.
      */
-    public function __construct(array $data = null)
+    public function __construct(?array $data = null)
     {
         $this->config = new Data($data ?: []);
         $this->setDefaults(new Data());