[BUGFIX] Fix PHP 8 warnings in GifBuilder

Added some guards, isset(), to ensure that array keys
that aren't defined are accessed and throws PHP warnings.

Resolves: #99845
Releases: main, 11.5
Change-Id: I009f5d23a9bf3cd112dc786256d1144983736dd3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78121


Reviewed-by: Ralph Brugger <typo3bugs@public.linkpool.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>