Route paths are empty when model is mounted at / #487

Steps to reproduce

1. Set up a model where the http.path property is set to /.
2. Open Loopback Explorer (the Swagger UI).
3. See that the page fails to load.

Current Behavior

The addRoute subfunction in the RestAdapter.allRoutes method has a bug where it doesn't account for the case where the path of a route is just /, so when the trailing slash removal code runs, it makes the path empty instead of keeping it at /.

Expected Behavior

If the path is /, the trailing slash removal code should leave it unchanged.

Additional information

linux x64 12.22.12
├── loopback@3.27.0
├── loopback-boot@2.28.0
├── loopback-component-explorer@6.5.1
├── loopback-component-storage@3.7.0
├── loopback-connector-postgresql@3.9.1