[BUGFIX] Fix PHP 8 warning in GeneralUtility

Handle faulty HTML tags missing an ending ' or ". This is
done by adding a missing Null coalescing operator.

Resolves: #99892
Releases: main, 11.5
Change-Id: I8a514f365699ab15fc0004141597e7e32d7ccb8c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77797


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