[BUGFIX] Fix undefined array key warning in ImageContentObject

Ensure the first argument passed to $this->cImage() is a
string if file is not defined in conf array.

Resolves: #96008
Releases: master
Change-Id: Icf84fd7fad66da4f248e3216ab4b79ac995a69c1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72200


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>