[BUGFIX] Fix undefined array key warning in GifBuilder

Resolves: #97211
Releases: main, 11.5
Change-Id: Ic0165c1c88bae6ef5206d79691aa5918762dc14c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73995


Tested-by: core-ci <typo3@b13.com>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>