[BUG] Dart-dio ignoring nullable references (nullableReferenceTypes: true)

[JamesFieldist]

Bug Report Checklist

• [X ] Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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

dart-dio doesn't honor nullable references types. Doesn't matter if nullableReferenceTypes: true or not.

openapi-generator version

7.14.0

OpenAPI declaration file content or url

"TacitQuestionDto": {
        "type": "object",
        "additionalProperties": false,
        "required": [
          "id",
          "primaryCategory",
          "secondaryCategory",
          "tertiaryCategory",
          "divisions",
          "question",
          "questionType",
          "frequency",
          "owningContact",
          "noOfLines",
          "options",
          "standAlone"
        ],
        "properties": {
          "id": {
            "type": "string",
            "description": "Id",
            "format": "uuid",
            "nullable": true
          },
          "primaryCategory": {
            "nullable": false,
            "$ref": "#/components/schemas/CategoryRefDto"
          },
          "secondaryCategory": {
            "nullable": false,
            "$ref": "#/components/schemas/CategoryRefDto"
          },
          "tertiaryCategory": {
            "nullable": true,
            "$ref": "#/components/schemas/CategoryRefDto",
            "x-nullable": true
          },
          "divisions": {
            "type": "array",
            "nullable": false,
            "items": {
              "$ref": "#/components/schemas/TacitDivisionDto"
            }
          },
          "descriptor": {
            "type": "string",
            "nullable": true
          },
          "order": {
            "type": "integer",
            "format": "int32",
            "nullable": true
          },
          "question": {
            "type": "string",
            "nullable": false
          },
          "questionType": {
            "nullable": false,
            "$ref": "#/components/schemas/AnswerTypes"
          },
          "noOfLines": {
            "type": "integer",
            "format": "int32",
            "nullable": false
          },
          "askDelayAfterNoAnswerDays": {
            "type": "integer",
            "format": "int32",
            "nullable": true
          },
          "frequency": {
            "nullable": false,
            "$ref": "#/components/schemas/TacitQuestionFrequency"
          },
          "options": {
            "type": "array",
            "nullable": false,
            "items": {
              "type": "string"
            }
          },
          "hiddenBy": {
            "type": "array",
            "nullable": true,
            "items": {
              "type": "string"
            }
          },
          "standAlone": {
            "type": "boolean",
            "nullable": false
          },
          "owningContact": {
            "nullable": false,
            "$ref": "#/components/schemas/ContactRefDto"
          },
          "followupQuestions": {
            "type": "array",
            "nullable": true,
            "items": {
              "$ref": "#/components/schemas/QuestionRefDto"
            }
          }
        }
      },

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please create a Gist (https://gist.github.com) or upload it somewhere else and
link it here.
-->

Generation Details

open-generator-config.yaml file:

generatorName: dart-dio
additionalProperties:
  pubName: api
  pubVersion: 0.0.1
  pubDescription: "API client for Dart"
  serializationLibrary: json_serializable
  enablePostProcessFile: true
  enumPropertyNaming: original
  useEnumExtension: true
  useSingleRequestParameter: true
  withSeparateModelsAndApi: true
  modelPropertyNaming: original
  supportNullable: true # Ensure OpenAPI nullable rules are respected
  legacyDiscriminatorBehavior: false
  strictNullChecks: true
  nullableReferenceTypes: true # If using C# generation

Steps to reproduce

Use any openapi document that has a nullable reference in it.

Run the cli like this:

			"@openapitools/openapi-generator-cli",
			"generate",
			"-i",
			"http://localhost:16049/help/v1/openapi.json",
			"-g",
			"dart-dio",
			"-c",
			"open-generator-config.yaml",
			"--enable-post-process-file"

The resulting property on the class generated will not be nullable and should be.

Related issues/PRs

Couldn't find any.

Suggest a fix

The dart generator(s) should honor nullable: true and/or x-nullable: true per the documentation on reference properties on schemas. As it stands right now it only honors nullable on concrete types, not references to other objects.