[HttpFoundation] fix return type declarations

Author: nicolas-grekas

Dumper/AbstractDumper.php

@@ -35,7 +35,7 @@ abstract class AbstractDumper implements DataDumperInterface, DumperInterface
     protected $indentPad = '  ';
     protected $flags;
 
-    private $charset;
+    private $charset = '';
 
     /**
      * @param callable|resource|string|null $output  A line dumper callable, an opened stream or an output path, defaults to static::$defaultOutput