Swift - 对JSON的值为nil的字典

Question

我有一个带有可选值的字典：

let user = [
  "name": "gui",
  "username": nil,
] as [String: Any?]

我只想转换为应该是{"name": "gui", username: null}

的json

但是，JSONSerialization.isValidJSONObject(user)会返回false，而try JSONSerialization.data(withJSONObject: user)会失败..

我应该如何处理？