[BUGFIX] Avoid fatal error in image rendering when processing fails

Fatal error can occur, when the file variable isn't a string,
which only happens, when file processing failed or the resolved
file isn't an image.

Nevertheless the fatal error should be avoided.

Also clean up some variable usage to not change the type
of variable and remove some not needed isset() check
for better readability.

Resolves: #91681
Releases: master, 10.4
Change-Id: Ibde8e1105fa7c0e233656b5dd49e083b6c7259ab
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64908


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Helmut Hummel <typo3@helhum.io>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Helmut Hummel <typo3@helhum.io>