what is EU going to do if Russia invades Poland anyway? Thermonuclear war? Are you saying the whole security apparatus and international institutions are all bluff? If so, Russia would have figured out the bluff by now.
I don’t claim to know the right solution to any difficulties in international politics, but I can say the difference between the current situation and your hypothetical is that Poland is in the EU so Article 42.7 of the Lisbon treaty world apply, and is also in NATO so Article 5 of the NATO treaty would apply too.
Nuclear deterrence doesn't mean that France would retaliate with nuclear weapons to a conventional attack, but that French nuclear weapons make it unlikely, not worth it, for Russia to use its own weapons first.
That law doesn’t prevent ~200 Russian cities from being evaporated by the French nuclear arsenal, and neither world it prevent France from responding to a nuclear attack with said arsenal.
https://www.europarl.europa.eu/meetdocs/2009_2014/documents/...
https://www.nato.int/cps/en/natohq/topics_110496.htm