I’ve always joked that it should be called a Kirky third, but only fellow Star Trek nerds get it, and it’s a terrible joke regardless.
There’s no official term for it because—unlike the Picardy third—it isn’t a generally used procedure. The original reason you hear minor-key music end with a major triad is that, in the Renaissance and Baroque, the minor triad generally wasn’t seen as harmonically stable enough to end an entire composition. You have to remember that even treating thirds in general as consonances was relatively new. They could find plenty of theoretical justification for this position based on the fact that a minor third above the fundamental isn’t found until a much higher partial in the overtone series (you have to go to at least the 19th partial to find a possible candidate). Minor triads were starting to be heard as stable enough for internal cadences, but not for final ones. One imagines that many composers wrote Picardy thirds simply because “it’s what one does at the end of a minor piece” rather than feeling any need to justify it at all.
So, although it tends to sound surprising to our ears, it would have sounded totally unremarkable during the Baroque. The opposite wasn’t done at all—and would have been heard as very strange—so no term was necessary. Although I’m sure composers have done it, especially post-modern ones, I’ve never come upon an inverse Picardy third despite studying a lot of music. If I came across one, I think “inverse Picardy third” would work nicely. I wouldn’t be at all surprised if some people have tried to coin a term for it, but with such a limited supply of examples I’d be surprised if it caught on. Maybe it will become a characteristic aspect of some composers, and it will come to be named after them or their region, much like one (so far unproven) explanation for the origin of the “Picardy third” term.