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 en

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 GH-11840.

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