[BUGFIX] Improve exception output for ImageService

Avoid spamming the log in PHP 7.2 due to get_class(null) if fetching the
image in ImageService->getImage() fails.

Resolves: #83570
Releases: master, 8.7, 7.6
Change-Id: Icc3c78f5b747842cee89d184134ff298b8e04a3a
Reviewed-on: https://review.typo3.org/55376


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>