[BUGFIX] PHP 8 warning: Undefined array key in DebugUtility

DebugUtility threw warnings of the type "Undefined array key"
which results in error for uncached content in frontend. This
warning is fixed in this patch.

Resolves: #95211
Releases: master
Change-Id: I692f88e1cf123003bfdbef4f8303c41fab855f01
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71079


Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>