[BUGFIX] Fix PHP warning in RTE parseFunc option prefixRelPathWith

Resolves: #98175
Releases: main, 11.5
Change-Id: Iccc439eafbbb1f959c8c2bc12248b7c4aa560260
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76466


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