toJson method
Implementation
Object? toJson(Object? Function(T? value) callback) {
final value = callback(this.value);
final entries = values.entries.map((e) {
final value = callback(e.value);
if (value == null) return null;
return MapEntry(e.key, value);
}).whereType<MapEntry<String, Object>>();
final x = Map.fromEntries(entries);
if (value != null) x['en'] = value;
if (x.length <= 1) return value;
return x;
}