[BUGFIX] Add fallback values for backend

Fix PHP8 related "undefined array key" warnings
which occur in the backend by adding the required
fallbacks.

Resolves: #95239
Releases: master
Change-Id: Id1477c95ac00e90b9d449271fb461f2b4dc34235
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71100


Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>