Merge branch '6.4' into 7.3

* 6.4:
  [ProxyManager] Remove extra space before return‑type colon in `proxy-implem.php`
  [Config] Use the empty string instead of null as an array offset

Conflicts:
 src/Symfony/Bridge/ProxyManager/Tests/LazyProxy/PhpDumper/Fixtures/proxy-implem.php
 src/Symfony/Bridge/ProxyManager/Tests/LazyProxy/PhpDumper/ProxyDumperTest.php

Author: GromNaN

Definition/Dumper/YamlReferenceDumper.php

@@ -240,6 +240,6 @@ private function getPrototypeChildren(PrototypedArrayNode $node): array
         }
         $keyNode->setInfo($info);
 
-        return [$key => $keyNode];
+        return [$key ?? '' => $keyNode];
     }
 }