[BUGFIX] Fix PHP 8 warning in RootlineUtility

Sanitize the mountPointParameter in class constructor.

Resolves: #99731
Releases: main, 11.5
Change-Id: Ic6fec228c462c7ec4a75a7efc934d7d17fc5c1e0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77657


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>