[TASK] Solve several phpstan issues in GifBuilder

Also add some (indirect) basic tests for
GifBuilder::calculateValue.

Used command:

> Build/Scripts/runTests.sh -s phpstanGenerateBaseline

Resolves: #97690
Releases: main, 11.5
Change-Id: I836db8570f64ecb2729c1707fcfe9fc1af672151
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74771


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