[BUGFIX] Avoid undefined array key warning

Return an empty array if the key does not exist.

Resolves: #96023
Releases: master, 11.5
Change-Id: I41cbea8c74ac91062c2e46efc16b4b6580be066b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72225


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>