[BUGFIX] Fix array undefined warnings in "DB Check" Module

Add fallback for undefined array keys to make "DB Check"
compatible with PHP8.

Also basic tests have been added to check each page in
the module.

Resolves: #94498
Releases: master
Change-Id: I98a598d0e809077cd89e7620efe31a498b3fbfa1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69759


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: crell <larry@garfieldtech.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>