[BUGFIX] Fix PHP Warning in GraphicalFunctions

Added a missing else case in getImageDimensions when image size
can't be determined. Return null instead of a boolean.

Resolves: #99998
Releases: main, 11.5
Change-Id: Idfe6bf49612392f66f1c3c60a47f2c463c6eb4ad
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77913


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