Merge branch '6.4' into 7.3

* 6.4:
  fix test setup
  [Validator] Review Turkish translations
  [Validator] Review Croatian translations
  [Validator] Review translations for Polish (pl)
  use the empty string instead of null as an array offset
  Review translations for Chinese (zh_TW)
  [Serializer] Adjust ObjectNormalizerTest for the accessor method changes from #61097

Author: xabbuh

Definition/Builder/ArrayNodeDefinition.php

@@ -347,7 +347,7 @@ public function normalizeKeys(bool $bool): static
 
     public function append(NodeDefinition $node): static
     {
-        $this->children[$node->name] = $node->setParent($this);
+        $this->children[$node->name ?? ''] = $node->setParent($this);
 
         return $this;
     }