HTML preview unit tests fail with libxml2 2.14.2

### Description

`tests.media.test_html_preview.OpenGraphFromHtmlTestCase.test_no_tree` no longer passes after upgrading to libxml2 2.14.2.

This is probably not Synapse's problem, filing here for the record.

### Steps to reproduce

- build Synapse against https://archive.archlinux.org/repos/2025/04/21/$repo/os/$arch (before libxml2 2.14.2 was uploaded)
- ⇒ observe tests passing
- build Synapse against https://archive.archlinux.org/repos/2025/05/01/$repo/os/$arch (after libxml2 2.14.2, but before python-lxml 5.4.0)
- ⇒ observe tests failing

### Homeserver

N/A

### Synapse Version

1.128.0+

### Installation Method

Other (please mention below)

### Database

N/A

### Workers

Single process

### Platform

Arch Linux (see above) / x86_64

### Configuration

_No response_

### Relevant log output

```shell
[FAIL]
Traceback (most recent call last):
File "/build/matrix-synapse/src/matrix-synapse/build/lib.linux-x86_64-cpython-313/tests/media/test_html_preview.py", line 329, in test_no_tree
self.assertIsNone(tree)
File "/usr/lib/python3.13/unittest/case.py", line 1303, in assertIsNone
self.fail(self._formatMessage(msg, standardMsg))
twisted.trial.unittest.FailTest: is not None

tests.media.test_html_preview.OpenGraphFromHtmlTestCase.test_no_tree
-------------------------------------------------------------------------------
Ran 4016 tests in 29.560s

FAILED (skips=181, failures=1, successes=3834)
```

### Anything else that would be useful to know?

Also filed for libxml2 as https://gitlab.gnome.org/GNOME/libxml2/-/issues/907.