input: Write a function to print the unique elements of a given list. If the given list is empty then return None.
  def print_unique(my_list):
      if not my_list:
          return None
      seen = set()
      for x in my_list:
          if x not in seen:
              print(x)
              seen.add(x)
  
  assert print_unique([1, 2, 2, 3, 4, 4])==None
  assert print_unique([5, 5, 6, 6, 7, 7])==None
  assert print_unique([1, 2, 3, 4, 5])==None
  assert print_unique([])==None
output: def print_unique(my_list):
      if not my_list:
          return None
      seen = set()
      for x in my_list:
          if x not in seen:
              print(x)
              seen.add(x)