fix(firebase_messaging): make notification body optional

- Updated FirebaseNotification model to make the body field optional
- Fixed a bug in FirebasePushNotificationClient where the notification body was incorrectly set to the payload title

Author: fulleni

lib/src/models/push_notification/firebase_request_body.dart

@@ -74,15 +74,15 @@ class FirebaseNotification extends Equatable {
   /// {@macro firebase_notification}
   const FirebaseNotification({
     required this.title,
-    required this.body,
+    this.body,
     this.image,
   });
 
   /// The notification's title.
   final String title;
 
   /// The notification's body text.
-  final String body;
+  final String? body;
 
   /// The URL of an image to be displayed in the notification.
   final String? image;

lib/src/services/firebase_push_notification_client.dart

@@ -116,7 +116,6 @@ class FirebasePushNotificationClient implements IPushNotificationClient {
           token: token,
           notification: FirebaseNotification(
             title: payload.title,
-            body: payload.title,
             image: payload.imageUrl,
           ),
           data: payload,