Thankfully, that has been thought of and, as much as possible, planned for.
How do you decrease the incentive for a preemptive strike? By building your nuclear arsenal so that it can survive one. There are 3 main things that help achieve this:
(1) Put your nukes somewhere where even a direct hit from an enemy nuke will not destroy them. That's why missile silos are underground.
(2) Hide your nukes. The enemy can't take out your nukes if they don't know where they are. Fixed-location nukes are bad at this. Mobility is good, but road and rail are not the best forms of mobility. Nukes in planes are better. Nukes in submarines are really good.
(3) Have lots of nukes. If the enemy can destroy 90% of your nukes with a preemptive strike but 10% of your nukes would still easily be enough to wipe them out, then a preemptive strike still doesn't achieve its goal. One way to increase nukes cheaply and effectively is with MIRVs (https://en.wikipedia.org/wiki/Multiple_independently_targeta...), which means one missile has multiple nuclear warheads that can independently hit different targets.
Basically, all this was thought out pretty thoroughly during the Cold War. The well-known acronym MAD stands for Mutually-Assured Destruction, and the importance of the assured part has not been overlooked.
Source: needed a government credit in college, took a class that was all about nuclear deterrence theory.
Once one Nuke is fired it's likely they'll all be because the only real defence against a nation using nuclear weapons against you is to try to wipe them out before they wipe you out.
In this situation hundreds of millions would be likely die directly from the attack and a similar amount would probably die from starvation shortly after as the global economy collapses and crops fail on mass.
There's just no situation that this is something we'd even want to consider. As much as I'd love NATO to be able to intervene it's just not possible. The best we can do is put pressure on Russia and show other nations that this is not acceptable and not in their national interest because of the harsh economic consequences.