Fix(typedRoutes): fixed StaticRoutes and DynamicRoutes being empty causing invalid syntax (#46620)

When `dynamicRouteTypes`/`staticRouteTypes` is empty, an invalid `link.d.ts` is generated. This just handles that case.

## 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)

packages/next/src/build/webpack/plugins/next-types-plugin.ts

@@ -196,7 +196,6 @@ function createRouteDefinitions(
     }
   }
 
-  const fallback = !edgeRoutes.length && !nodeRoutes.length ? 'string' : ''
   const routes = [...edgeRoutes, ...nodeRoutes, ...extraRoutes]
 
   // By exposing the static route types separately as string literals,
@@ -267,10 +266,12 @@ declare namespace __next_route_internal_types__ {
   type OptionalCatchAllSlug<S extends string> =
     S extends \`\${string}\${SearchOrHash}\` ? never : S
 
-  type StaticRoutes = ${staticRouteTypes}
-  type DynamicRoutes<T extends string = string> = ${dynamicRouteTypes}
+  type StaticRoutes = ${staticRouteTypes || 'string'}
+  type DynamicRoutes<T extends string = string> = ${
+    dynamicRouteTypes || 'string'
+  }
 
-  type RouteImpl<T> = ${fallback}
+  type RouteImpl<T> =
     | StaticRoutes
     | \`\${StaticRoutes}\${Suffix}\`
     | (T extends \`\${DynamicRoutes<infer _>}\${Suffix}\` ? T : never)