chore: fix typing

Author: ChiragAgg5k

templates/dart/lib/src/exception.dart.twig

@@ -9,7 +9,7 @@ class {{spec.title | caseUcfirst}}Exception implements Exception {
   /// for more information.
   final String? type;
   final int? code;
-  final String response;
+  final String? response;
 
   /// Initializes an {{spec.title | caseUcfirst}} Exception.
   {{spec.title | caseUcfirst}}Exception([this.message = "", this.code, this.type, this.response]);