Merge branch '6.3' into 6.4

* 6.3:
  [Serializer] Fix `@method` annotation
  fix compatibility with Doctrine DBAL 4
  ensure string type with mbstring func overloading enabled
  [HttpKernel] Fix quotes expectations in tests
  [Validator] updated Greek translation
  [Cache][HttpFoundation][Lock] Fix empty username/password for PDO PostgreSQL
  [HttpClient][WebProfilerBundle] Do not generate cURL command when files are uploaded

Author: nicolas-grekas

HeaderUtils.php

@@ -256,7 +256,7 @@ public static function parseQuery(string $query, bool $ignoreBrackets = false, s
     private static function groupParts(array $matches, string $separators, bool $first = true): array
     {
         $separator = $separators[0];
-        $separators = substr($separators, 1);
+        $separators = substr($separators, 1) ?: '';
         $i = 0;
 
         if ('' === $separators && !$first) {

Session/Storage/Handler/PdoSessionHandler.php

@@ -90,12 +90,12 @@ class PdoSessionHandler extends AbstractSessionHandler
     /**
      * Username when lazy-connect.
      */
-    private string $username = '';
+    private ?string $username = null;
 
     /**
      * Password when lazy-connect.
      */
-    private string $password = '';
+    private ?string $password = null;
 
     /**
      * Connection options when lazy-connect.