[BUGFIX] Access to not defined array key in install tool

In the install tool context $GLOBALS['BE_USER'] is not available
and any access must be avoided especially from PHP 8.0.

Resolves: #94418
Releases: master
Change-Id: I1df52aedd9ba1149e4009c9cc3a6707ba114c607
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69576


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