[BUGFIX] Check if category images exist before import

Resolves: #63935
Change-Id: I94e18e205f7266cadefb16250c04ad62ee9646e2
Reviewed-on: http://review.typo3.org/35617
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>