[BUGFIX] Avoid uninitialized string offset in ContentObjectRenderer

Resolves: #99073
Releases: main, 11.5
Change-Id: Ib4d69ebd3258451af19616810730ecd33c31077d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77322


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>