Merge branch '6.0' into 6.1

* 6.0:
  [Security][AbstractToken] getUserIdentifier() must return a string
  [HttpFoundation] Prevent accepted rate limits with no remaining token to be preferred over denied ones
  Email image parts: regex for single closing quote
  [Serializer] Throw InvalidArgumentException if the data needed in the constructor doesn't belong to a backedEnum

Author: xabbuh

Authentication/Token/AbstractToken.php

@@ -48,7 +48,7 @@ public function getRoleNames(): array
 
     public function getUserIdentifier(): string
     {
-        return $this->user->getUserIdentifier();
+        return $this->user ? $this->user->getUserIdentifier() : '';
     }
 
     /**