[BUG] [Dart] Optional number properties aren't being parsed as null

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

When a property with a number type is set on a component schema and isn't set as a required key, it causes a parsing error at runtime when that value isn't in the json or is null.

Specifically, the error is in the fromJson() method when num.parse() is called, which expects the value to be non-null.

Workaround: add "nullable": true onto the property in the spec, but ideally properties should be treated as optional by default since the OpenAPI 3.0 spec says that they are here.

openapi-generator version

Library version: ^6.1.0
Flutter version: 3.29.0

OpenAPI declaration file content or url

{
  "openapi": "3.0.0",
  "info": {
    "title": "Sample API",
    "version": "1.0.0"
  },
  "paths": {
    "/test": {
      "get": {
        "responses": {
          "200": {
            "description": "Success",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/Foo"
                }}}}}}}},
  "components": {
    "schemas": {
      "Foo": {
        "type": "object",
        "properties": {
          "bar": {
            "type": "number"
          }}}}}}

Generation Details

java -jar openapi-generator-cli-7.12.0.jar generate -i openapi.json -g dart -o test-openapi

Steps to reproduce

Execute shell command written above, using the support files included above, then check the generated dart code in .\test-openapi\lib\model\foo.dart.

Should produce this:

...

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

  ///
  /// Please note: This property should have been non-nullable! Since the specification file
  /// does not include a default value (using the "default:" property), however, the generated
  /// source code must fall back to having a nullable type.
  /// Consider adding a "default:" property in the specification file to hide this note.
  ///
  num? bar;

  ...

  /// Returns a new [Foo] instance and imports its values from
  /// [value] if it's a [Map], null otherwise.
  // ignore: prefer_constructors_over_static_methods
  static Foo? fromJson(dynamic value) {
    if (value is Map) {
      final json = value.cast<String, dynamic>();

      // Ensure that the map contains the required keys.
      // Note 1: the values aren't checked for validity beyond being non-null.
      // Note 2: this code is stripped in release mode!
      assert(() {
        requiredKeys.forEach((key) {
          assert(json.containsKey(key), 'Required key "Foo[$key]" is missing from JSON.');
          assert(json[key] != null, 'Required key "Foo[$key]" has a null value in JSON.');
        });
        return true;
      }());

      return Foo(
        bar: num.parse('${json[r'bar']}'), // <<<<<<<<<<<<<<<<<<<<<<<--- runtime error here
      );
    }
    return null;
  }

  ...
}

Related issues/PRs

https://github.com/gibahjoe/openapi-generator-dart/issues/181

Suggest a fix

Perhaps something like this:

return Foo(
  bar: json[r'bar'] == null ? null : num.parse('${json[r'bar']}'),
);

or

return Foo(
  bar: num.tryParse('${json[r'bar']}'),
);

Run the Java OpenAPI Generator command and inspect test-openapi/lib/model/foo.dart, especially Foo.fromJson(). Reproduce the missing or null bar value with the supplied OpenAPI declaration; done means generated Dart parsing accepts the optional number without a runtime error while preserving required-key checks.