Make ClassToString print null values

[ebelevics]

Make ClassToString print null values

[BreX900]

@ebelevics You can import 'package:class_to_string/class_to_flat_string.dart'; and have a custom ClassToString creator set up.

void main() {
  ClassToString.creator = (className, types) => NullableClassToIndentString(className, types ?? const []);

  // ...
}

class NullableClassToIndentString extends ClassToIndentString {
  NullableClassToIndentString(super.className, [super.types = const []]);
  
  void add(String field, Object? value) => add(field, value ?? 'null');
}

I'm considering adding a flag on the yaml file or a flag on ClassToString.
I'm also considering moving the stringify_type: params | fields option from the yaml file to the ClassToString class because the toString method is very subjective/personal so it should be customizable based on the developer and not on the package designer
I have to evaluate better