I’m not sure how any votes are “nullified.” Under the Constitution, the only people who get to vote for President are the electors. While presidential candidates may appear on your ballot, you are voting for a slate of electors who may or may not be purportedly bound to vote for that particular candidate. You don’t have a vote for president that can be “nullified.”
In early Presidential elections, the presidential candidates’ names did not even appear on the ballot, if there was any sort of popular vote at all. Electors were appointed by the state legislature in several states. State legislatures could go back to that today if they wanted to. Or, as arrogantsob said, they could choose electors by lottery or any other way they wish.
Some states’ laws purport to bind electors to vote for a particular candidate. But it is not at all clear whether those laws are enforceable under the Constitution. Of course, it is also unclear whether a law binding an elector to vote for the “national popular vote” winner would be enforceable. What would stop one of those Pennsylvania electors from ignoring the Pennsylvania law and just voting for whoever he wants to?
If the electors are that empowered and can vote for whomever they want, then I don’t understand why citizens even bother to vote. If an elector can cast a ballot for anyone other than that state’s voters vote for, what’s the point of election for the office of president?