[BUGFIX] Allow direct access to BE again while being logged in

Having a valid backend authentication and accessing /typo3
now redirects to the backend view again.

Resolves: #84991
Releases: master
Change-Id: I483f6408192a7a239144868048476057ca5b282c
Reviewed-on: https://review.typo3.org/56949


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>