60. implement UndefinedToNull<T>

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

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