60. implement UndefinedToNull<T>

  - accepted / - tried

Following the coding question 176. undefined to null on BFE.dev, please implement UndefinedToNull<T>.

type A = UndefinedToNull<string>    // string
type B = UndefinedToNull<undefined> // null
type C = UndefinedToNull<[undefined, null]> // [null, null]
type D = UndefinedToNull<{
  a: undefined,
  b: [1, undefined]
}> // {a: null, b: [1, null]}

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