[BUGFIX] Fix PHP 8 warning in TreeDataProvider

Resolves: #99315
Releases: main, 11.5
Change-Id: Ib6bae3be83742212d063bd7b746bb2885d79222b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77560


Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>