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, 3, 2, 4, 5, 1])==None
  assert print_unique([4, 4, 4, 4])==None
  assert print_unique([1, 2, 3, 4, 5])==None
  assert print_unique([])==None
output: def print_unique(lst):
      if not lst:
          return None
      unique_elements = list(set(lst))
      print(unique_elements)