siblings() raises warnings on an element with no parent #139

Description of the bug

siblings() reads $m->parentNode->childNodes without checking for a parent, so calling it on an element that has no parent raises two warnings:

$doc  = qp('<?xml version="1.0"?><root/>')->get(0)->ownerDocument;
$lone = $doc->createElement('lone');

qp($lone)->siblings()->count();
// Warning: Attempt to read property "childNodes" on null in src/Helpers/QueryFilters.php on line 1083
// Warning: foreach() argument must be of type array|object, null given in src/Helpers/QueryFilters.php on line 1083
// 0

The result (an empty set) is right, and matches jQuery 3.7.1 ($(document.createElement('lone')).siblings().length is 0). But the warnings are wrong, and become a fatal error under any error handler that converts warnings to exceptions (PHPUnit, Laravel, Symfony, ...). PHP 7.x raises the equivalent notice and warning.

An element in a detached subtree, which does have a parent, is fine: qp($a)->siblings() returns its siblings.

Expected

siblings() on an element with no parent returns an empty set, without warnings. contents() already guards with if (empty($m->childNodes)) continue;; siblings() needs the same check on $m->parentNode.

Found while reviewing #129, which fixed the same class of problem in parentsUntil() (an infinite loop on a detached element) by routing it through one walk helper. siblings() keeps its own loop there, so it was not covered.

QueryPath version

main at ed98764, PHP 8.5.12.