input: Write a function to find the union of two lists. The union of two lists is a set that contains all the unique elements from both lists. If either of the lists is empty, the function should return an empty set.
  assert unionList([1, 2, 3, 4, 5], [3, 4, 5, 6, 7])=={1, 2, 3, 4, 5, 6, 7}
  assert unionList([], [3, 4, 5, 6, 7])==set()
  assert unionList([1, 2, 3], [])=={1, 2, 3}
  assert unionList([], [])==set()
output: def unionList(list1, list2):
      if not list1 or not list2:
          return set()
      return set(list1).union(set(list2))