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