fix(typed-routes): Fix route type fallback (#73271)

### What?

This PR updates the fallback type for typed routes to match the one in
https://github.com/vercel/next.js/blob/canary/packages/next/src/types.ts#L58.

### Why?

During builds, the `Route` type is initialized as the fallback type
before being populated by the routes. This causes temporary type
warnings in the project, since a route like `"" | Route` becomes `"" |
string` which can raise an ts-eslint error like `"" is overridden by
string in this union type`.
This becomes a problem in both failed builds, and in CI where a linter
and a build can run in parallel. Updating the type to match the stub
type fixes the issue.

Co-authored-by: Sam Ko <[email protected]>
Co-authored-by: JJ Kasper <[email protected]>

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

@@ -394,9 +394,9 @@ function createRouteDefinitions() {
     dynamicRouteTypes += pluginState.routeTypes[type].dynamic
   }
 
-  // If both StaticRoutes and DynamicRoutes are empty, fallback to type 'string'.
+  // If both StaticRoutes and DynamicRoutes are empty, fallback to type 'string & {}'.
   const routeTypesFallback =
-    !staticRouteTypes && !dynamicRouteTypes ? 'string' : ''
+    !staticRouteTypes && !dynamicRouteTypes ? 'string & {}' : ''
 
   return `// Type definitions for Next.js routes