Merge branch '7.4' into 8.0

* 7.4:
  [Lock][DynamoDB] Allow symfony/lock 8.0
  [ProxyManager] Remove extra space before return‑type colon in `proxy-implem.php`
  [Config] Use the empty string instead of null as an array offset
  [Messenger] Support signing messages per handler

Author: GromNaN

Definition/Dumper/YamlReferenceDumper.php

@@ -239,6 +239,6 @@ private function getPrototypeChildren(PrototypedArrayNode $node): array
         }
         $keyNode->setInfo($info);
 
-        return [$key => $keyNode];
+        return [$key ?? '' => $keyNode];
     }
 }