[BUG] [Dart] Array of array items don't handle required correctly

Created by: 0xNF

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

Double arrays will generate with invalid defaults in the event of null values encountered during deserialization

This will generate the following invalid deserializer:

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

  /* omitted */ 

  List<List<int>> subarray;
  static WithNonNullableArray? fromJson(dynamic value) {
    if (value is Map) {
      final json = value.cast<String, dynamic>();
     /* omitted */
      return WithNonNullableArray(
        subarray: json[r'subarray'] is List
          ? (json[r'subarray'] as List).map(
              (e) => e == null ? null : (e as List).cast<int>()
            ).toList()
          : null,
      );
    }
    return null;
  }
}

Null isn't valid here, and so becomes a syntax error. The proper typing of the defaults for this example should be <int>[]

openapi-generator version

Latest commit as of 9/18/22 (https://github.com/OpenAPITools/openapi-generator/commit/863dbc7c3ee39af1ac931d2949388f7f84896ec4)

OpenAPI declaration file content or url

Specfile

openapi: 3.0.3
info:
  version: "1.1"
  title: Dart Uint8list Demo
servers:
  - url: "localhost"
    variables:
      host:
        default: localhost
paths:
  /item:
    get:
      operationId: GetItem
      description: "Should return an Item"
      responses:
        "200":
          description: items
          content:
            application/json:
              schema:
                $ref: "#components/schemas/WithNonNullableArray"
components:
  schemas:
    WithNonNullableArray:
      type: object
      required: [subarray]
      properties:
        subarray:
          type: array
          minItems: 12
          maxItems: 12
          items:
            $ref: "#/components/schemas/SubArray"
    WithNullableArray:
      type: object
      required: [subarray]
      properties:
        months:
          type: array
          nullable: true
          items:
            $ref: "#/components/schemas/SubArray"
    SubArray:
      type: array
      items:
        type: integer
        format: int32

Suggest a fix

Changing the native_class.mustache file to include

(e) => e == null ? {{#required}}{{#isNullable}}null{{/isNullable}}{{^isNullable}}const <{{items.items.dataType}}>[]{{/isNullable}}{{/required}}{{^required}}null{{/required}}

Changes the output to be more appropriate:

      return WithNonNullableArray(
        subarray: json[r'subarray'] is List
          ? (json[r'subarray'] as List).map(
              (e) => e == null ? const <int>[] : (e as List).cast<int>()
            ).toList()
          :  const <int>[],
      );