[BUG] [dart-dio] Generated ToJson methods for models throws exception for json_serializable

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• 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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The generator produces invalid Dart code for nullable Map properties, causing compilation errors in serialization methods (ToJson) when using json_serializable.

openapi-generator version

7.10.0

OpenAPI declaration file content or url

type: object
required:
  - subject
properties:
  subject:
    type: string
  data:
    type: object
    additionalProperties:
      type: string

Steps to reproduce

Generated model ToJson code will look like that, while getting the following compilation error:

Error: Expected ')' before this.
      if (instance.data case final value?) 'data': value,
                        ^^^^

Map<String, dynamic> _$UsersMobileNotificationToJson(
        UsersMobileNotification instance) =>
    <String, dynamic>{
      'subject': instance.subject,
      if (instance.data case final value?) 'data': value,
    };

Suggest a fix

Looks like it is caused by latest update of json_serializable since its version isn't enforced.

Start with the dart-dio generator and reproduce the issue using the OpenAPI declaration in the report with json_serializable enabled. Inspect the generated model's ToJson method and compare it with the reported compilation error. Done means the nullable Map model serializes to valid Dart code and the generated output compiles.