[BUGFIX] Do not reprocess image preview when empty configuration is passed

Fixes regression introduced in https://review.typo3.org/53399
Where thumbnail was regenerated every time process was called with
empty configuration like:

$fileObject->process(ProcessedFile::CONTEXT_IMAGEPREVIEW, [])

Resolves: #83242
Relates: #81776
Releases: master, 8.7, 7.6
Change-Id: I6a4c331bad4d80225d3914f1a48f6071b0d87380
Reviewed-on: https://review.typo3.org/55040


Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>