[Config] Use the empty string instead of null as an array offset

Author: santysisi

Definition/Dumper/YamlReferenceDumper.php

@@ -249,6 +249,6 @@ private function getPrototypeChildren(PrototypedArrayNode $node): array
         }
         $keyNode->setInfo($info);
 
-        return [$key => $keyNode];
+        return [$key ?? '' => $keyNode];
     }
 }