[BUGFIX] Fix undefined array key warnings in GifBuilder

Resolves: #104639
Releases: main, 12.4, 11.5
Change-Id: I74a0cd9e856283d26a382582cc5d6b8e99887cd4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85694


Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>