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