hide release nodes that don't have a file (except in issue queue)

this patch allows us to have a release node in the issue queue, but not visible as something on the release listings for your project. e.g. we could add a "6.x-dev" release node as a placeholder (it'll become the HEAD releases in a little while) that was already available in the issue queue, but it wouldn't show up in http://drupal.org/node/3060/release, wouldn't be a valid choice for the default version on http://drupal.org/node/3060/edit and so on.

slick, huh? ;)