[php-src] PR #23888: Fix GH-23887: HTMLCollection::namedItem() mishandles the first child

Pull Request: https://github.com/php/php-src/pull/23888 Author: lazerg dom_html_collection_named_item() seeds the iterator with basep->children, but the handlers don't agree on what that seed means. The noop handler (used by getElementsByClassName('')) never moves it, so the first child is checked forever: that's the assertion in the report, an infinite loop in release builds, or a wrong match if the first child has the key as its id. The class name handler treats the seed as already visited and skips the first child. The children handler never advances past the first child when it is an element that doesn't match, so it hangs. The iterator now starts with a NULL candidate, which the class name and noop handlers already handle, and the children and tag name handlers start from basep->children when they see NULL. Fixes #23887