[TASK] Avoid undefined variables/array access in admin panel

Fixes both admin panel and time tracker
array access warnings with PHP8.

Resolves: #95309
Releases: master
Change-Id: I528c7dcb0f0daa1bd898dc342b5047120e27932d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71174


Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>