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