provide empty data map to render and render-resource

I'm not sure if this is something you'd want to allow or not. I've used some other mustache template libs, in other languages, that allowed this. Anyway, I'm also new to Clojure so this may or may not be the best way to go about this. This was just a possible way to do it that I saw.

This adds another overloaded `render` and `render-resource` that simply provide an empty map if they're only passed in a single parameter.