I think NATO would respond, but non-nuclear. There is no need to answer tactical nuclear weapons with a nuclear strike. There are tons of NATO air forces available to strike against Russian troops in Ukrainian territory. They would just obliterate any Russian troops there.

It was a small but interesting news item, that a few days before the invasion, some of the US stealth bombers were moved to Rammstein air force base. So at any time there would be the capability of a forceful answer. But this also shows how ridiculous the claims were, that the NATO would seek to attack Russia, as that is the thing the NATO has tried hard to avoid so far.