[BUGFIX] Handle missing BE_USER in ext manager

The FileHandlingUtility may be used from a context
where there is no BE_USER.
Avoid a warning in PHP 8

Resolves: #97204
Releases: main, 11.5
Change-Id: Iae4d2d77cc69f46811933b6b081044256a1c7ce2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73955


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>