fix: LinkTo with incomplete model failing in rendering tests

LinkTo needs route context to allow omitting model from current active
route. Without the guard, tests where LinkTo rendered in tests without
routing transition started will break. See issues/19364

packages/@ember/-internals/glimmer/tests/integration/components/link-to/rendering-angle-test.js

@@ -126,9 +126,7 @@ moduleFor(
           });
         });
       });
-      debugger;
       return this.visit('/parents/1').then(() => {
-        debugger;
         this.assertText('Link To Child');
       });
     }
@@ -173,7 +171,6 @@ moduleFor(
 
     ['@test should be able to be inserted in DOM when router is setup but not started']() {
       this.render(`<LinkTo @route="dynamicWithChild.child">Link</LinkTo>`);
-      debugger;
       this.assertComponentElement(this.element.firstChild, {
         tagName: 'a',
         content: 'Link',

packages/@ember/-internals/routing/lib/services/routing.ts

@@ -54,8 +54,10 @@ export default class RoutingService extends Service {
 
   generateURL(routeName: string, models: {}[], queryParams: {}) {
     let router = this.router;
-    // return early when the router microlib is not present, which is the case for {{link-to}} in integration tests
-    if (!router._routerMicrolib) {
+    // return early when the router microlib is not present, which is the case for <LinkTo/> in integration tests
+    // also return early when transition has not started, when rendering in tests without visit(),
+    // we cannot infer the route context which <LinkTo/> needs be aware of
+    if (!router._routerMicrolib || !router._initialTransitionStarted) {
       return;
     }
 

packages/@ember/-internals/routing/lib/system/router.ts

@@ -130,6 +130,7 @@ class EmberRouter extends EmberObject {
   rootURL!: string;
   _routerMicrolib!: Router<Route>;
   _didSetupRouter = false;
+  _initialTransitionStarted = false;
 
   currentURL: string | null = null;
   currentRouteName: string | null = null;
@@ -414,6 +415,7 @@ class EmberRouter extends EmberObject {
       if (initialTransition && initialTransition.error) {
         throw initialTransition.error;
       }
+      this._initialTransitionStarted = true;
     }
   }
 
@@ -621,6 +623,7 @@ class EmberRouter extends EmberObject {
    */
   reset() {
     this._didSetupRouter = false;
+    this._initialTransitionStarted = false;
     if (this._routerMicrolib) {
       this._routerMicrolib.reset();
     }