If no hero image is set an error is thrown when viewing the gallery (handlebars).

If you create a gallery but don't add a hero image an error is thrown in the cloudinaryURL helper.

It assumes that the context will have a url attribute but if there is no hero image the context is an empty object.

This fix just checks that a url attribute exists and if not returns null.
I feel this isn't the best solution but I don't know enough about helpers to fix it properly!