input: Write a python function to merge two lists and return a sorted list.
  assert union_list([1, 5, 3], [2, 6, 4]) == [1, 2, 3, 4, 5, 6]
  assert union_list([10, 20, 5], [15, 25, 0]) == [0, 5, 10, 15, 20, 25]
  assert union_list([], [1, 2, 3]) == [1, 2, 3]
output: def union_list(list1, list2):
      merged_list = list1 + list2
      return sorted(merged_list)