Added "If Image Exists"

Through the tutorial, it was necessary for every blog post to have an image associated with it. And if the blog post didn't, it would show a broken link. But, thanks to some fancy PHP and a twig extension, I added an "if asset_exists" statement to the twig so now the image is only shown if it exists, and not otherwise.

Signed-off-by: Parth Sarin <parthsarin@gmail.com>