[BUGFIX] Avoid str_replace(..., null) in ContentObjectRenderer

Calling str_replace() with second argument non-string is
deprecated as of PHP 8.1. Sanitize stdWrap_brTag() in
ContentObjectRenderer to avoid a case found by unit tests.

Resolves: #95775
Releases: master
Change-Id: I9353ec28d23ec3b0ef50b8ecf48053ce27f5645e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71973


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>