[BUGFIX] Use correct fallback types in ImageDimension

Replace wrong null fallback for "width" and "height"
with empty string and also cast the values before
passing to GraphicalFunctions->getImageScale()
which expects them to be string.

Resolves: #97952
Releases: main, 11.5
Change-Id: I3c2eafd89f81f0b5d606a1c936c43a8bca57659c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75197


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>