If Ukraine had nukes and NATO didn't want them to use them or risk losing them, they may have acted.
Again, I think it's unlikely and the "Russia wouldn't have invited Ukraine if they had kept their nukes" doesn't actually fly (and probably would just have been another reason for Russia to invade: 'they're right there and they have nukes!'
But the general thought is that NATO would have had to protect them (I don't buy it, but that's the theory).