Make it work for Rails 6 and above; fix a couple of nil errors

[bulgakke]

1. In Rails 6+, ActionDispatch::Routing::ConsoleFormatter is a module, not a class (if I understood correctly, the Sheet class retains previous functionality; I got some empty strings in testing, not sure if it's because of some formatter change or just my wierd routes)
2. When the route prefixes are not set (i. e. when using non-resourceful routes and not assigning prefixes manually), previous version led to nil errors and (if you ignore nils, e. g. with using string interpolation instead on concatenating) (_path) annotations, so I return an empty string instead whenever a route doesn't have a prefix.

I wonder why this gem (or any gem like it) doesn't see development, seems like a useful tool to me.