[BUG] [Dart] schema property of type: array and nullable: true with no default value should use null in constructor

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

openapi-generator version

commit 8511ce360c7f9166828bf71a4fd61fad107b63c1, May 7

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  version: "1.1"
  title: Dart Nullable List
servers:
  - url: 'localhost'
    variables:
      host:
        default: localhost
components:
  schemas:
    ItemWithNullableArray:
      type: object
      properties:
        nullableList:
          type: array
          nullable: true
          items:
            type: string
paths:
  /items:
    get:
      operationId: ItemWithNullableArray
      responses:
        "200":
          description: A list of ItemWithNullableArray
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ItemWithNullableArray'

Generation Details

The output for lists in Dart uses the default const [], but if no default value is supplied, and the property is nullable, it should probably be null instead.

class ItemWithNullableArray {
  /// Returns a new [ItemWithNullableArray] instance.
  ItemWithNullableArray({
    this.nullableList = const [],
  });

  List<String>? nullableList;
}

instead it'd be nice to get this:

class ItemWithNullableArray {
  /// Returns a new [ItemWithNullableArray] instance.
  ItemWithNullableArray({
    this.nullableList,
  });

  List<String>? nullableList;
}

Otherwise, until support for OAS 3.1 comes along, there's no way to make default-null containers in Dart, despite the requirements of certain API specifications.

Steps to reproduce

1. openapi-generator.jar generate -i spec.yaml -g dart -o nullable

Related issues/PRs

Suggest a fix

1. Adjust the Mustache templates to generate null for the combination of isContainer && isNullable && !defaultValue

Run the provided OpenAPI declaration with openapi-generator.jar generate -i spec.yaml -g dart -o nullable and inspect the generated constructor. Start with the Dart Mustache templates, focusing on the isContainer, isNullable, and missing-default combination. Done means a nullable array without a default generates a null constructor value rather than const [], while other list defaults remain unchanged.