fix(onesignal): make notification contents optional

- Remove requirement for contents in OneSignalRequestBody
- Update OneSignalPushNotificationClient to use payload.message instead of payload.title for contents

Author: fulleni

lib/src/models/push_notification/onesignal_request_body.dart

@@ -20,8 +20,8 @@ class OneSignalRequestBody extends Equatable {
     required this.appId,
     required this.includePlayerIds,
     required this.headings,
-    required this.contents,
     required this.data,
+    this.contents,
     this.bigPicture,
   });
 
@@ -35,7 +35,7 @@ class OneSignalRequestBody extends Equatable {
   final Map<String, String> headings;
 
   /// The notification's content.
-  final Map<String, String> contents;
+  final Map<String, String>? contents;
 
   /// The custom data payload
   final PushNotificationPayload data;

lib/src/models/push_notification/onesignal_request_body.g.dart

@@ -103,7 +103,6 @@ class OneSignalPushNotificationClient implements IPushNotificationClient {
       appId: appId,
       includePlayerIds: deviceTokens,
       headings: {'en': payload.title},
-      contents: {'en': payload.title},
       bigPicture: payload.imageUrl,
       data: payload,
     );