[BUGFIX] Prevent PHP warning in FolderBrowser

Resolves: #96083
Releases: master, 11.5
Change-Id: I76d486d1b817832cbf7c62252fe4a5e3f9f94391
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72289


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Bartsch <bo@cedev.de>