Allow path providers that don't add path elements each time

This PR allow path providers to don't add path elements on each `RouteStack` objects.
This simply allow an empty path for `RouteStack`s.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | None.
| License       | MIT
| Doc PR        | None. But could be great to create a doc about custom path providers.