It would be business as usual for all other countries. Pronouncements wohld be made in the media but no action. This is not unique to Ukraine.

It won't nuke Ukraine.

Firstly, in the Russia you need a triple order to launch nukes. Putin's order is not enough. And the two other people would not want to start a nuclear war. There's no way they'd okay a nuclear strike. And if Putin tried to convince them of that, they'd consider Putin having crossed a line that really does matter. And then there would be a headless Putin standing and a Putinless head rolling on the floor, really soon after that. Putin won't try his luck with that. He knows he'll just die if he does.

And secondly, it's unlikely that any of the nukes work. It's the same as with military trucks' tires. Everybody assumed they'd be never used in reality, so all the maintenance resources were embezzled.

Even if the Russia was to somehow assume that some of the nukes are not duds, they'd understand that they cannot know which one is the working one. You launch 1000 missiles, of which 3 detonate, and they happen to be the ones hitting less important targets. What happens next? Heh.