[BUG] [DART] Dart Generator does not generate Multipart properly. Sends an empty object instead of file

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue? Started from 6.0.0 (5.4.0 worked perfectly)
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

The Dart generator does not generate multipart properly. It sends an empty object instead of a multipart file provided.
It appears only with such configuration: dart-dio, json_serializable and timemachine.

If you use just a dart generator instead of dart-dio it generates it properly.

Here is an example of faulty outputs

Future<Response<UploadProfessionalImage201Response>> uploadProfessionalImage({ 
    required String professionalId,
    MultipartFile? image,
    CancelToken? cancelToken,
    Map<String, dynamic>? headers,
    Map<String, dynamic>? extra,
    ValidateStatus? validateStatus,
    ProgressCallback? onSendProgress,
    ProgressCallback? onReceiveProgress,
  }) async {
    final _path = r'/images/professionals/{professionalId}/'.replaceAll('{' r'professionalId' '}', professionalId.toString());
    final _options = Options(
      method: r'POST',
      headers: <String, dynamic>{
        ...?headers,
      },
      extra: <String, dynamic>{
        'secure': <Map<String, String>>[
          {
            'type': 'apiKey',
            'name': 'BearerToken',
            'keyName': 'Authorization',
            'where': 'header',
          },
        ],
        ...?extra,
      },
      contentType: 'multipart/form-data',
      validateStatus: validateStatus,
    );

    dynamic _bodyData;

    try {

    } catch(error, stackTrace) {
      throw DioError(
         requestOptions: _options.compose(
          _dio.options,
          _path,
        ),
        type: DioErrorType.other,
        error: error,
      )..stackTrace = stackTrace;
    }

    final _response = await _dio.request<Object>(
      _path,
      data: _bodyData,
      options: _options,
      cancelToken: cancelToken,
      onSendProgress: onSendProgress,
      onReceiveProgress: onReceiveProgress,
    );

    UploadProfessionalImage201Response _responseData;

    try {
_responseData = deserialize<UploadProfessionalImage201Response, UploadProfessionalImage201Response>(_response.data!, 'UploadProfessionalImage201Response', growable: true);
    } catch (error, stackTrace) {
      throw DioError(
        requestOptions: _response.requestOptions,
        response: _response,
        type: DioErrorType.other,
        error: error,
      )..stackTrace = stackTrace;
    }

    return Response<UploadProfessionalImage201Response>(
      data: _responseData,
      headers: _response.headers,
      isRedirect: _response.isRedirect,
      requestOptions: _response.requestOptions,
      redirects: _response.redirects,
      statusCode: _response.statusCode,
      statusMessage: _response.statusMessage,
      extra: _response.extra,
    );
  }

As you can see _bodyData is completely empty.

This is a problem because it makes this API call completely useless and swapping to standard dart generator would mean a lot of refactoring and losing features for us.

openapi-generator version

From 6.0.0 and onward

5.4.0 works fine

OpenAPI declaration file content or url

 "/images/professionals/{professionalId}/": {
            "post": {
                "security": [
                    {
                        "BearerToken": []
                    }
                ],
                "description": "Upload image for professional (profile image)",
                "consumes": [
                    "multipart/form-data"
                ],
                "tags": [
                    "Image"
                ],
                "operationId": "UploadProfessionalImage",
                "parameters": [
                    {
                        "type": "file",
                        "description": "Image file",
                        "name": "image",
                        "in": "formData",
                        "required": false
                    },
                    {
                        "type": "string",
                        "description": "Professional ID",
                        "name": "professionalId",
                        "in": "path",
                        "required": true
                    }
                ],
                "responses": {
                    "201": {
                        "description": "Created",
                        "schema": {
                            "allOf": [
                                {
                                    "type": "object"
                                },
                                {
                                    "type": "object",
                                    "properties": {
                                        "data": {
                                            "$ref": "#/definitions/dto.Image"
                                        }
                                    }
                                }
                            ]
                        }
                    },
                    "default": {
                        "description": "",
                        "schema": {
                            "allOf": [
                                {
                                    "type": "object"
                                },
                                {
                                    "type": "object",
                                    "properties": {
                                        "error": {
                                            "$ref": "#/definitions/errs.E"
                                        }
                                    }
                                }
                            ]
                        }
                    }
                }
            }
        },

Generation Details

openapi-generator-cli generate -i swagger.json -g dart-dio -o ./ -p dateLibrary=timemachine,pubLibrary=eliva.api,pubName=project_api,serializationLibrary=json_serializable

Steps to reproduce

Just generate using the command and the faulty code appears

Suggest a fix

No idea unfortunatelly

Reproduce the issue with the supplied OpenAPI declaration and the openapi-generator-cli command using dart-dio, json_serializable, and timemachine. Compare the generated uploadProfessionalImage method with the working dart generator and the 5.4.0 output. Done means the generated dart-dio method includes the provided MultipartFile in the multipart request body rather than leaving _bodyData empty.