Merge branch '6.4' into 7.0

* 6.4:
  Update Pbkdf2PasswordHasher.php
  Bump Symfony version to 6.3.5
  Update VERSION for 6.3.4
  Update CHANGELOG for 6.3.4
  Bump Symfony version to 5.4.29
  Update VERSION for 5.4.28
  Update CONTRIBUTORS for 5.4.28
  Update CHANGELOG for 5.4.28
  Fixed attachment base64 content string in MailerSendApiTransport
  [Security] Prevent creating session in stateless firewalls
  [Serializer] Fix union of enum denormalization
  [Serializer] Fix wrong InvalidArgumentException
  Fix breaking change in AccessTokenAuthenticator

Author: derrabus

Hasher/Pbkdf2PasswordHasher.php

@@ -69,7 +69,7 @@ public function hash(#[\SensitiveParameter] string $plainPassword, string $salt
             throw new LogicException(sprintf('The algorithm "%s" is not supported.', $this->algorithm));
         }
 
-        $digest = hash_pbkdf2($this->algorithm, $plainPassword, $salt, $this->iterations, $this->length, true);
+        $digest = hash_pbkdf2($this->algorithm, $plainPassword, $salt ?? '', $this->iterations, $this->length, true);
 
         return $this->encodeHashAsBase64 ? base64_encode($digest) : bin2hex($digest);
     }