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

[Lymdun]

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?
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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.