input: Write a function to find the union of two lists. The union of two lists is a list that contains all the unique elements present in both lists, without duplicates. If any of the lists is empty, the union should contain only the elements of the non-empty list.
  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):
      return set(list1).union(set(list2))