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, 2, 5, 1, 4])==(1, 5)
  assert get_max_min([-1, -5, 3, 2])==(-5, 3)
  assert get_max_min([10])==(10, 10)
  assert get_max_min([])==(None, None)
output: def get_max_min(lst):
      if not lst:
          return None, None
      return min(lst), max(lst)