fix(aio): allow empty titles for contents pages

Now if you specifically provide an empty `@title` tag
for a contents file, the doc-gen will not complain and
the browser will just display a reasonable default.

aio/src/app/app.component.spec.ts

@@ -272,6 +272,14 @@ describe('AppComponent', () => {
       fixture.detectChanges();
       expect(titleService.setTitle).toHaveBeenCalledWith('Angular - Pipes');
     });
+
+    it('should update the document title, with a default value if the document has no title', () => {
+      const titleService = TestBed.get(Title);
+      spyOn(titleService, 'setTitle');
+      locationService.go('file-not-found');
+      fixture.detectChanges();
+      expect(titleService.setTitle).toHaveBeenCalledWith('Angular');
+    });
   });
 
   describe('autoScrolling', () => {
@@ -424,12 +432,18 @@ class TestHttp {
     "contents": "<h1>Test Doc</h1>"
   };
 
+  fileNotFoundDoc = {
+    "title": "",
+    "contents": "Page not found"
+  };
+
   // get = jasmine.createSpy('get').and.callFake((url: string) => { ... });
   get(url: string) {
     const json =
       /navigation.json/.test(url) ? this.navJson :
       /api/.test(url) ? this.apiDoc :
       /pipes/.test(url) ? this.pipesDoc :
+      /file-not-found/.test(url) ? this.fileNotFoundDoc :
       this.testDoc;
     return of({ json: () => json });
   }

aio/src/app/app.component.ts

@@ -150,6 +150,10 @@ export class AppComponent implements OnInit {
   }
 
   setDocumentTitle(title) {
-    this.titleService.setTitle(`Angular - ${title}`);
+    if (title.trim()) {
+      this.titleService.setTitle(`Angular - ${title}`);
+    } else {
+      this.titleService.setTitle('Angular');
+    }
   }
 }