[TASK] Avoid undefined key warning in rte editor

Array access with undefined key emits undefined index warning.
This patch sanitize the calls to mitigate these warnings.

Releases: master, 11.5
Resolves: #96072
Change-Id: Ic86781728fcb61b4dc3d9442fe5228c6b8a81474
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72282


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