Don't use empty <img> tags on pages with no images

http://caa.mbsandbox.org/release/e00b9eca-3f69-4757-b59a-f61779069482 has no cover art at all. Despite that, the page loads with an inital empty <img> tag which means that anyone with JS turned off sees a broken image when no image should be shown at all. Everyone else briefly sees a broken image until the JS has checked that there is (still) no cover art and removes it.

Since we already keep track of whether a release has cover art or not, I can't see any point to doing it this way. When we know there is no cover art for a release, we should just output the "no cover art" message in the first place. It would also make the page rendering a lot smoother, right now the broken image being replaced by text is rather distracting to me.