[BUGFIX beta] Fix link-to throwing in integration tests

Fixes #15831

Fixes a regression introduced by #15788 by returning early again from `routing.generateURL()` when `router._routerMicrolib` is not present.

packages/ember-routing/lib/services/routing.js

@@ -48,13 +48,16 @@ export default Service.extend({
   },
 
   generateURL(routeName, models, queryParams) {
+    let router = get(this, 'router');
+    if (!router._routerMicrolib) { return; }
+
     let visibleQueryParams = {};
     if (queryParams) {
       assign(visibleQueryParams, queryParams);
       this.normalizeQueryParams(routeName, models, visibleQueryParams);
     }
 
-    return get(this, 'router').generate(routeName, ...models, { queryParams: visibleQueryParams });
+    return router.generate(routeName, ...models, { queryParams: visibleQueryParams });
   },
 
   isActiveForRoute(contexts, queryParams, routeName, routerState, isCurrentWhenSpecified) {