Letter Combinations of a Phone Number

This problem is part of solulu Pro

Given a string of digits 2–9, return every letter string it could represent on a phone keypad (2 → abc, 3 → def, 4 → ghi, 5 → jkl, 6 → mno, 7 → pqrs, 8 → tuv, 9 → wxyz), in any order. An empty input gives an empty list.