I don't think that's ever actually been a rule. There is a rule that powers generally don't go to war if they can't win, or if the cost is greater than the gain. You can see how a face-off between two Nuclear Powers would fall under this rule.
The question would be whether North Korea having a few nukes, of less-than-intercontinental range, would be enough to gain them protection under the rule. Or at least enough for them to be able to extort concessions.
Also, war has been avoided for over fifty years without North Korea having nukes. It's not clear why they suddenly need nukes now, if the status quo is their goal anyway.