How to model a data object that represents a PATCH request body whose fields can be undefined, null or values

Let's suppose I need to call an HTTP API PATCH endpoint whose schema for the body is:

{  ..."type": "object","properties": {"propertyOne": { "type": ["string", "null"] },"propertyTwo": { "type": "string" }  }}

According to the schema above, all of the following bodies are valid:

{ "propertyOne": null }

{ "propertyTwo": "test" }

{ "propertyOne": "test", "propertyTwo": "test" }

How do I model such a data object in Dart so that when I call .toJson on that object, it outputs a Map with propertyOne set to null if the property was explicitly set to null, with propertyOne set to a string if the property was initialized with a string or excludes propertyOne if no value was specified when initializing the class?

I'm currently using freezed and have a working model for the request if the schema of the API wouldn't include null as an allowed value. How I model that is:

@freezedclass PatchRequestBody with _$PatchRequestBody {  const factory PatchRequestBody({      @JsonKey(includeIfNull: false) String? propertyOne,      @JsonKey(includeIfNull: false) String? propertyTwo    }) = _PatchRequestBody;  factory PatchRequestBody.fromJson(Map<String, dynamic> json) =>      _$PatchRequestBodyFromJson(json);}

The issue comes when I want different behaviours for when a value is specifically set to null versus when a value is not specified.

For the case above, if I run PatchRequestBody(propertyOne: "test").toJson() I get { "propertyOne": "test" } which is great. If I run PatchRequestBody(propertyOne: null).toJson() it outputs an empty object, which is not great but can be fixed by removing the @JsonKey(includeIfNull: false) annotation. This unfortunately makes the case where I run PatchRequestBody(propertyTwo: "test").toJson() output { "propertyOne": null, "propertyTwo": "test" } which is not what I desire.

I tried looking for some solutions and found this Optional class from a package called quiver but it seems like they advise against not using it anymore.

Otherwise I couldn't really find a way to solve or model this.