[dart] Default value is only available when nullable is not set

Created by: agilob

Dart json_serializable does not allow setting nullable: false AND defaultValue at the same time.

As nullable: true with default value doesn't make sense, neither nullable: false it requires removal of nullable.

It throws exception:

Error with `@JsonKey` on `mapProperty`. Cannot use `defaultValue` on a field with `nullable` false.
package:openapi/model/additional_properties_class.dart:31:23
   ╷
31 │   Map<String, String> mapProperty;
   │                       ^^^^^^^^^^^

@kuhnroyal