[BUGFIX] Fix PHP 8 warning in ContentObjectRenderer

Handle faulty HTML tags that can cause a PHP warning.
This is done by adding a isset() guard.

Resolves: #99896
Releases: main, 11.5
Change-Id: I69c03fa55d7ad3d3f355bf76c43b3fff6e0a1f85
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77812


Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>