input: Write a function to print the unique elements of a given list. If the given list is empty then return None. 
  assert print_unique([1, 2, 2, 3, 4, 4]) == None
  assert print_unique([5, 5, 6, 7, 7, 8]) == None
  assert print_unique([]) == None
output: def print_unique(lst):
      if not lst:
          return None
      unique_elements = list(set(lst))
      print(unique_elements)