[Dart-dio-next] nullable fields sent when using multipart forms

This is what is generated today with last version:

Future<Response<User>> saveProfile({ 
    String? email,
    String? firstName,
    String? lang,
    String? lastName,
    String? mobile,
    String? password,
    MultipartFile? avatar,
    CancelToken? cancelToken,
    Map<String, dynamic>? headers,
    Map<String, dynamic>? extra,
    ValidateStatus? validateStatus,
    ProgressCallback? onSendProgress,
    ProgressCallback? onReceiveProgress,
  }) 
....

_bodyData = FormData.fromMap(<String, dynamic>{
        r'email': encodeFormParameter(_serializers, email, const FullType(String)),
        r'firstName': encodeFormParameter(_serializers, firstName, const FullType(String)),
        r'lang': encodeFormParameter(_serializers, lang, const FullType(String)),
        r'lastName': encodeFormParameter(_serializers, lastName, const FullType(String)),
        r'mobile': encodeFormParameter(_serializers, mobile, const FullType(String)),
        r'password': encodeFormParameter(_serializers, password, const FullType(String)),
        r'avatar': avatar,
      });

Problem with that is that null fields are still sent to the payload, and even worse encodeFormParameter is forcing null field to be empty strings.

Basically here I don't want an empty string sent as a new password ^^, so I'm getting 400 all the time because password is not correct.

I guess an easy fix would be to wrap then into a if like this:

_bodyData = FormData.fromMap(<String, dynamic>{
        if(email != null)  r'email': encodeFormParameter(_serializers, email, const FullType(String)),
        if(firstName != null)  r'firstName': encodeFormParameter(_serializers, firstName, const FullType(String)),
        if(lang != null) r'lang': encodeFormParameter(_serializers, lang, const FullType(String)),
        if(lastName != null) r'lastName': encodeFormParameter(_serializers, lastName, const FullType(String)),
        if(mobile != null) r'mobile': encodeFormParameter(_serializers, mobile, const FullType(String)),
        if(password != null) r'password': encodeFormParameter(_serializers, password, const FullType(String)),
        if(avatar != null) r'avatar': avatar,
      });

Start with the Dart-dio-next generated saveProfile method and trace how encodeFormParameter is used in the FormData.fromMap payload. Verify how nullable multipart parameters are represented and compare that with the requested conditional entries. Done means omitted nullable fields are not sent while non-null fields, including avatar, remain in the payload without being converted to empty strings.