https://docs.microsoft.com/ja-jp/dotnet/api/system.text.json.serialization.jsonconverter-1.handlenull?view=net-10.0
JsonConverterT.HandleNull Property (System.Text.Json.Serialization) | Microsoft Learn
Gets a value that indicates whether null should be passed to the converter on serialization, and whether Null should be passed on deserialization.
property systemjson serializationtextmicrosoftlearn