[BUGFIX] Fix undefined array key warning in HtmlParser

Resolves: #97095
Releases: main, 11.5
Change-Id: I783dab0b2d16d8071a2ecd061083a580f77533b2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73780


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>