JsonConvert.SerializeObject(model, new JsonSerializerSettings {
 NullValueHandling = NullValueHandling.Ignore 
})

未过滤null序列化结果
"memberquery": {
  "name": "test",
  "sex": "女",
  "phonenumber": Null
  "birthdate": Null
}


已过滤null序列化结果
"memberquery": {
  "name": "test",
  "sex": "女",
}