chore: update example app for VertexAI (#13128)

Author: Lyokone

packages/firebase_vertexai/firebase_vertexai/example/lib/main.dart

@@ -12,11 +12,14 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+import 'package:firebase_core/firebase_core.dart';
+import 'package:firebase_vertexai/firebase_vertexai.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_markdown/flutter_markdown.dart';
-import 'package:firebase_vertexai/firebase_vertexai.dart';
-import 'package:firebase_core/firebase_core.dart';
+
+// REQUIRED if you want to run on Web
+const FirebaseOptions? options = null;
 
 void main() {
   runApp(const GenerativeAISample());
@@ -137,7 +140,8 @@ class _ChatWidgetState extends State<ChatWidget> {
   );
 
   Future<void> initFirebase() async {
-    await Firebase.initializeApp();
+    // ignore: avoid_redundant_argument_values
+    await Firebase.initializeApp(options: options);
   }
 
   void _scrollDown() {