update user model

packages/stream_chat/lib/src/models/user.g.dart

@@ -9,7 +9,7 @@ part of 'user.dart';
 User _$UserFromJson(Map<String, dynamic> json) {
   return User(
     id: json['id'] as String,
-    role: json['role'] as String? ?? '',
+    role: json['role'] as String?,
     createdAt: json['created_at'] == null
         ? null
         : DateTime.parse(json['created_at'] as String),