Fix EZP-23287: Empty Image FieldType triggers twig_error_runtime

check if image is set, otherwise don't try to render the image

(cherry picked from commit 93db5acadfc4cdc81cdb284bad0f4d97bda4596b)
Fix EZP-23287: Output img tag for alt text on invalid image alias

Yes, it is a thing: http://stackoverflow.com/questions/5775469/whats-the-valid-way-to-include-an-image-with-no-src
Merge pull request #990 from ezsystems/fix_EZP-23287_emptyImage

Fix EZP-23287: empty Image FieldType triggers twig_error_runtime