fix: payin/payout deserialization (#11)

* properly handle when `requiredPaymentDetails` is null

* make constructor private

Author: ethanwlee

lib/src/http_client/tbdex_http_client.dart

@@ -14,6 +14,8 @@ import 'package:typeid/typeid.dart';
 import 'package:web5/web5.dart';
 
 class TbdexHttpClient {
+  TbdexHttpClient._();
+
   static const _jsonHeader = 'application/json';
   static const _expirationDuration = Duration(minutes: 5);
 

lib/src/protocol/models/resource_data.dart

@@ -145,7 +145,9 @@ class PayinMethod {
       name: json['name'],
       description: json['description'],
       group: json['group'],
-      requiredPaymentDetails: JsonSchema.create(json['requiredPaymentDetails']),
+      requiredPaymentDetails: json['requiredPaymentDetails'] != null
+          ? JsonSchema.create(json['requiredPaymentDetails'])
+          : null,
       fee: json['fee'],
       min: json['min'],
       max: json['max'],
@@ -199,7 +201,9 @@ class PayoutMethod {
       name: json['name'],
       description: json['description'],
       group: json['group'],
-      requiredPaymentDetails: JsonSchema.create(json['requiredPaymentDetails']),
+      requiredPaymentDetails: json['requiredPaymentDetails'] != null
+          ? JsonSchema.create(json['requiredPaymentDetails'])
+          : null,
       fee: json['fee'],
       min: json['min'],
       max: json['max'],