Solves Bug PIMCORE-2112 & removes warning in debug-mode

Commit 8e9d178:
http://www.pimcore.org/issues/browse/PIMCORE-2112

Commit 3f72818:
If debug mode is enabled a php warning is printed for every image that does not exist.

Warning: filesize(): stat failed for /var/www/htdocs/dev/website/var/assets/test.jpg in /var/www/htdocs/dev/pimcore/lib/Pimcore/Image/Adapter/Imagick.php on line 59

So I think it would be better to first check if imagemagick can read the image and second if the filesize is 0.