[Config][XmlReferenceDumper] Prevent potential \TypeError

src/Symfony/Component/Config/Definition/Dumper/XmlReferenceDumper.php

@@ -91,7 +91,7 @@ private function writeNode(NodeInterface $node, int $depth = 0, bool $root = fal
                 }
 
                 if ($prototype instanceof PrototypedArrayNode) {
-                    $prototype->setName($key);
+                    $prototype->setName($key ?: '');
                     $children = [$key => $prototype];
                 } elseif ($prototype instanceof ArrayNode) {
                     $children = $prototype->getChildren();