Is it possible to distinguish between absent (no value) and null when serializing?

Let's say I have a SimpleValue class with a nullableString:

```dart
abstract class SimpleValue implements Built {
@BuiltValueSerializer(serializeNulls: true)
static Serializer get serializer => _$simpleValueSerializer;

String? get nullableString;

factory SimpleValue([void Function(SimpleValueBuilder) updates]) = _$SimpleValue;
SimpleValue._();
}
```

I would like to be able to have this behaviour when serializing to JSON:

```dart
final noValue = SimpleValue();
print(serializers.toJson(SimpleValue.serializer, value)); // should print {}

final nullValue = SimpleValue((value) => value.nullableString = null);
print(serializers.toJson(SimpleValue.serializer, nullValue)); // should print { "nullableString": null }
```

Is it possible?