[BUGFIX] Add guard clause while checking if user is system maintainer

System maintainers must be admin users which should be checked with a
guard clause. This avoids showing the modules to editors whose id is in
the list of system maintainers.

Resolves: #82616
Releases: master, 9.5
Change-Id: I84cd20d2b84eaa0182c27943ffdcde005980a426
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63832


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Alexander Schnitzler <git@alexanderschnitzler.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Alexander Schnitzler <git@alexanderschnitzler.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>