input: Write a function to find the union of two lists. It should return a set containing all unique elements from both lists. If either list is empty, it should return the non-empty list as a set. If both lists are empty, it 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]) == {3, 4, 5, 6, 7}
  assert unionList([1, 2, 3, 4, 5], []) == {1, 2, 3, 4, 5}
  assert unionList([], []) == set()
output: def unionList(list1, list2):
      if not list1:
          return set(list2)
      if not list2:
          return set(list1)
      return set(list1 + list2)