Fix json_encode result on DOMDocument

According to https://www.php.net/manual/en/class.domdocument:
  When using json_encode() on a DOMDocument object the result will be
  that of encoding an empty object.

But this was broken in 8.1. The output was `{"config": null}`.
That's because the config property is defined with a default value of
NULL, hence it was included. The other properties are not included
because they don't have a default property, and nothing is ever written
to their backing field. Hence, the JSON encoder excludes them.
Similarly, `(array) $doc` would yield the same `config` key in the
array.

Closes phpGH-11840.

NEWS

@@ -13,6 +13,7 @@ PHP                                                                        NEWS
   . Fix empty argument cases for DOMParentNode methods. (nielsdos)
   . Fixed bug GH-11791 (Wrong default value of DOMDocument::xmlStandalone).
     (nielsdos)
+  . Fix json_encode result on DOMDocument. (nielsdos)
 
 - Core:
   . Fixed oss-fuzz #60741 (Leak in open_basedir). (ilutov)

ext/dom/php_dom.stub.php

@@ -707,7 +707,7 @@ class DOMDocument extends DOMNode implements DOMParentNode
      * @readonly
      * @deprecated
      */
-    public mixed $config = null;
+    public mixed $config;
 
     public bool $formatOutput;
 

ext/dom/tests/DOMDocument_json_encode.phpt

@@ -0,0 +1,11 @@
+--TEST--
+JSON encoding a DOMDocument
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$doc = new DOMDocument;
+echo json_encode($doc);
+?>
+--EXPECT--
+{}