Today I learned that Congress never bothered to pass a law clarifying what’s to be done under Section 4 of the Twentieth Amendment.

Per Wikipedia:

Section 4 permits Congress to statutorily clarify what should occur if either the House of Representatives must elect the president, and one of the candidates from whom it may choose dies, or if the Senate must elect the vice president and one of the candidates from whom it may choose dies. Congress has never enacted such a statute.