input: Write a function to return the maximum and minimum values in a given list. If the list is empty, return (None, None).
  assert get_max_min([1, 2, 3, 4, 5])==(1, 5)
  assert get_max_min([7, 2, 9, 4, 6])==(2, 9)
  assert get_max_min([-10, 0, 10, 20, -20])==(-20, 20)
  assert get_max_min([])==(None, None)
output: def get_max_min(lst):
      if not lst:
          return (None, None)
      return (min(lst), max(lst))