Article 50 doesn't require a free trade deal to exist after the Brexit, the "no deal" scenario is what happens if Britain exits without a replacement deal, and it's considered (by both sides) an outcome that's bad for UK.
Article 50 means that no deal happens without unanimity in the EU. The only way that binds the EU is in limiting the ability to make a deal, not obligating them to accept a unilateral UK demand.
So what happens if the EU and UK don't reach an agreement before the article 50 time limit expires? It doesn't make sense that the UK just gets to do what they like, otherwise they wouldn't need to negotiate at all.
As I say in other comments, WTO rules automatically apply - to both sides. The EU is fully bound by its own laws. It cannot unilaterally decide to reject WTO rules.