[BUGFIX] Avoid undefined array key warnings in GifBuilder

GIFBUILDER processing doesn't return a processedFile

Resolves: #97651
Releases: main, 11.5
Change-Id: I24964ec640636dc88b10a5433517bed0ce576ed0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74754


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