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