allow initialization with an empty route collection

This allows initialization of the `Poet` object with an empty route collection, e.g.

```javascript
var poet = Poet(app, { 
  posts: '/posts',
  routes: {}
});
```

Currently it just (silently) throws an error and fail to populate the `poet.posts` collection.

This is useful if you want to use poet but use your own custom routes. The `addRoute` method doesn't work in this case, because that only allows you to override, not eliminate routes.

Even if that's not a very common use case, this PR still prevents a `null` error.