replace CPP with non-nullable property

Using constructor property promotion for the $metaFile property does not
make much sense for two reasons:

* We need to declare it nullable just for the sake of the constructor
  accepting null while the property itself will always receive a string.
* The initially set value is immediately overwritten in the constructor.

Author: xabbuh

src/Symfony/Component/Config/ResourceCheckerConfigCache.php

@@ -23,6 +23,8 @@
  */
 class ResourceCheckerConfigCache implements ConfigCacheInterface
 {
+    private string $metaFile;
+
     /**
      * @param string                                    $file             The absolute cache path
      * @param iterable<mixed, ResourceCheckerInterface> $resourceCheckers The ResourceCheckers to use for the freshness check
@@ -31,7 +33,7 @@ class ResourceCheckerConfigCache implements ConfigCacheInterface
     public function __construct(
         private string $file,
         private iterable $resourceCheckers = [],
-        private ?string $metaFile = null,
+        ?string $metaFile = null,
     ) {
         $this->metaFile = $metaFile ?? $file.'.meta';
     }