Fix json_encode result on DOMDocument

[nielsdos]

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.

[Girgias]

I suppose this might be a BC in that if someone access the value without loading a document it will now throw an Error. But then I don't see how this is likely...

[nielsdos]

I suppose this might be a BC in that if someone access the value without loading a document it will now throw an Error.

No it won't, with this PR:

<?php
$doc = new DOMDocument;
var_dump($doc->config);
echo json_encode($doc), "\n";

outputs:

NULL
{}

DOM properties always go through getters & setters, their backing storage is unused.
Reflection could be affected though, i.e. ReflectionProperty::hasDefaultValue() will now return false instead of true (but ReflectionProperty::getDefaultValue() will still return NULL).

[Girgias]

Right, then LGTM