They won't use one, not because of Ukraine, not because of NATO and not because Putin doesn't want to. China is the country Putin depends upon and which he will listen to; and Xi will tell him not to launch such a missile.
Why?
Because a nuclear exchange may get out of hand. And a nuclear wind anywhere in the northern hemisphere will affect everyone else. And China doesn't want that any more than Europe does.
So it ain't going to happen.