Fix(typedRoutes): fixed empty DynamicRoutes or StaticRoutes causing Route to turn into type string (#46980)
Fixes #46974
When `dynamicRouteTypes` or `staticRouteTypes` is empty, type `DynamicRoutes`/`StaticRoutes` fallbacks to type `string`, causing type `Route` to resolve to type `string` as well, meaning that the user can use any string as `href`. This fixes that by making those types fallback to type `never` instead, and if both `dynamicRouteTypes` and `staticRouteTypes` are empty, fallback to type `string`.
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] [e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs) tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)