`documentFragment.firstChild` returns null if first child is e.g. `<tr></tr>`

[danieldiekmeier]

Describe the bug

When you put a standalone <tr></tr> element in a <template> and try to get it out with .firstChild, you get null instead. This also happens with <tbody> or <td>, but NOT with <li>.

To Reproduce

import { GlobalRegistrator } from '@happy-dom/global-registrator';
GlobalRegistrator.register();

const html = '<tr></tr>'

const elem = document.createElement('template');
elem.innerHTML = html;
const fragment = elem.content;

console.log(fragment.firstChild)

This prints null.

If you use something like const html = '<div></div>' instead, it returns the <div> node.

Expected behavior

When you run this code in the browser, it returns the node <tr></tr>:

Device:

System:
  OS: macOS 15.0.1
  CPU: (10) arm64 Apple M1 Max
  Memory: 9.37 GB / 64.00 GB
  Shell: 5.9 - /bin/zsh
Binaries:
  Node: 22.13.0 - ~/.local/share/mise/installs/node/22/bin/node
  npm: 10.9.2 - ~/.local/share/mise/installs/node/22/bin/npm
  pnpm: 9.12.3 - ~/.local/share/mise/installs/node/22/bin/pnpm
npmPackages:
  @happy-dom/global-registrator: ^16.5.3 => 16.5.3
  happy-dom: ^16.5.3 => 16.5.3

Additional context
Add any other context about the problem here.