use the empty string instead of null as an array offset

Author: xabbuh

Definition/Builder/ArrayNodeDefinition.php

@@ -331,7 +331,7 @@ 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;
     }