[BUG] [DART] The constructors of generated nested class declarations do not instantiate fields.

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue? yes
• Have you validated the input using an OpenAPI validator (example)? yes
• What's the version of OpenAPI Generator used? v4.2.2
• Have you search for related issues/PRs? yes
• What's the actual output vs expected output? I expect classes in the generated model directory to have a non-empty constructor that also instantiates fields of the class. Some generated classes are embedded into other generated classes as fields. When such a class is instantiated, those fields of this class, that have a type of other generated classes do not get automatically instantiated. The consequence is that, when you want to use such a class, then you need to instantiate not just the class itself but also MANUALLY all its fields that do not get automatically instantiated. It's even worse, when such an object in a field has also fields,that need to be instantiated. I expect all generated classes to have a non-empty constructor, that automatically instantiates all fields that have a type of a generated class.

openapi-generator version

v4.2.2

OpenAPI declaration file content or url

My openapi.json specification:

{
    "openapi": "3.0.2",
    "info": {
        "title": "test",
        "version": "v1"
    },
    "paths": {
        "/api/v1/average/height/": {
            "post": {
                "tags": [
                    "height",
                    "average"
                ],
                "summary": "GetAverageHeight",
                "description": "Getaverageheightatthespecifiedage.",
                "operationId": "get_average_height_api_v1_average_height__post",
                "requestBody": {
                    "content": {
                        "application/json": {
                            "schema": {
                                "$ref": "#/components/schemas/Person"
                            }
                        }
                    },
                    "required": true
                },
                "responses": {
                    "200": {
                        "description": "SuccessfulResponse",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "$ref": "#/components/schemas/PersonHeight"
                                }
                            }
                        }
                    },
                    "422": {
                        "description": "ValidationError",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "$ref": "#/components/schemas/HTTPValidationError"
                                }
                            }
                        }
                    }
                }
            }
        }
    },
    "components": {
        "schemas": {
            "AgeValue": {
                "title": "AgeValue",
                "required": [
                    "unit",
                    "value"
                ],
                "type": "object",
                "properties": {
                    "unit": {
                        "title": "Unit",
                        "enum": [
                            "month"
                        ],
                        "type": "string"
                    },
                    "value": {
                        "title": "Value",
                        "maximum": 216.0,
                        "minimum": 0.0,
                        "type": "number",
                        "description": "Ageofperson."
                    }
                }
            },
            "HTTPValidationError": {
                "title": "HTTPValidationError",
                "type": "object",
                "properties": {
                    "detail": {
                        "title": "Detail",
                        "type": "array",
                        "items": {
                            "$ref": "#/components/schemas/ValidationError"
                        }
                    }
                }
            },
            "HeightValue": {
                "title": "HeightValue",
                "required": [
                    "unit",
                    "value"
                ],
                "type": "object",
                "properties": {
                    "unit": {
                        "title": "Unit",
                        "enum": [
                            "cm"
                        ],
                        "type": "string"
                    },
                    "value": {
                        "title": "Value",
                        "maximum": 300.0,
                        "minimum": 10.0,
                        "type": "number",
                        "description": "Valueoftheheight."
                    }
                }
            },
            "Person": {
                "title": "Person",
                "required": [
                    "age"
                ],
                "type": "object",
                "properties": {
                    "age": {
                        "$ref": "#/components/schemas/AgeValue"
                    }
                }
            },
            "PersonHeight": {
                "title": "PersonHeight",
                "required": [
                    "person",
                    "height"
                ],
                "type": "object",
                "properties": {
                    "person": {
                        "$ref": "#/components/schemas/Person"
                    },
                    "height": {
                        "$ref": "#/components/schemas/HeightValue"
                    }
                }
            },
            "ValidationError": {
                "title": "ValidationError",
                "required": [
                    "loc",
                    "msg",
                    "type"
                ],
                "type": "object",
                "properties": {
                    "loc": {
                        "title": "Location",
                        "type": "array",
                        "items": {
                            "type": "string"
                        }
                    },
                    "msg": {
                        "title": "Message",
                        "type": "string"
                    },
                    "type": {
                        "title": "ErrorType",
                        "type": "string"
                    }
                }
            }
        }
    },
    "servers": [
        {
            "url": "http://10.0.2.2:8000",
            "description": "Justatestserver."
        }
    ]
}

My flutterconfig-dart.json file:

{
    "browserClient": false,
    "useEnumExtension": true
}

Command line used for generation

java -jar .\openapi-generator-cli-4.2.2.jar generate -i .\openapi.json -g dart -o .\openapi-test -c flutterconfig-dart.json

Steps to reproduce

Just generate the code using the command line, and then open any model dart files, such as:
\openapi-test\lib\model\person.dart

There you can see that the constructor of the Person class is Person(); So no instantiation of the field age takes place.

So in order to use the Person class you would need to do:

Person person = new Person();
person.age = new AgeValue(); // This would be totally unnecessary, with the fix I suggest below.

Imagine how big this problem is, if you wanted to instantiate a class with deeply nested field of other classes.

Suggest a fix

Extend all generated constructors to instantiate all fields.

Reproduce the issue with the provided OpenAPI specification, flutterconfig-dart.json, and generator command, then inspect openapi-test/lib/model/person.dart. Confirm that Person() leaves its age field uninstantiated, including the nested model relationships shown in the specification. Done means generated Dart constructors instantiate fields whose types are generated classes.