[BUGFIX] Properly validate start module

Fix incorrect handling of result from call to method
$this->moduleLoader->checkMod(). The method has string|array|bool
as return value. The array value is returned in case a valid module
where access is granted exists. The check should be adapted to
reflect this.

Resolves: #100040
Releases: 11.5
Change-Id: Ibb52febfb7a9f87f4bec31eb87478507b78564ca
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77975


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>