[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/55370


Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Mattia D <mattia.dorigatti@brandnamic.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Benni Mack <benni@typo3.org>