[BUGFIX] Prevent undefined array key access in modAccess()

This fixes a couple of undefined array key accesses
in BackendUserAuthentication->modAccess().

Besides, the code is a bit simplified and some
comments are added / fixed.

Resolves: #95560
Releases: master
Change-Id: I0fb20e1b9783fb770ff344bc0b4dc339f4a2ad50
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71529


Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>