dynamite: Fix non-required request bodies

packages/dynamite/lib/src/openapi_builder.dart

@@ -652,19 +652,20 @@ class OpenAPIBuilder implements Builder {
                                 );
                                 switch (mimeType) {
                                   case 'application/json':
+                                    final nullable = _isParameterNullable(
+                                      operation.requestBody!.required,
+                                      mediaType.schema?.default_,
+                                    );
                                     b.optionalParameters.add(
                                       Parameter(
                                         (final b) => b
                                           ..name = _toDartName(result.name)
-                                          ..type = refer(result.name)
+                                          ..type = refer(_makeNullable(result.name, nullable))
                                           ..named = true
                                           ..required = operation.requestBody!.required ?? false,
                                       ),
                                     );
-                                    final nullable = _isParameterNullable(
-                                      operation.requestBody!.required,
-                                      mediaType.schema?.default_,
-                                    );
+
                                     if (nullable) {
                                       code.write('if (${_toDartName(result.name)} != null) {');
                                     }