input: Write a function to calculate the minimum and maximum values in a given array. If the array is empty, return None, None.
  assert min_max([3, 4, 5, 1, 2])==(1, 5)
  assert min_max([3, 5, 10, 1, 2])==(1, 10)
  assert min_max([4, 3, 1, 2])==(1, 4)
  assert min_max([3, 5, 4, 1, 2])==(1, 5)
  assert min_max([])==(None, None)
output: def min_max(arr):
      if not arr:
          return None, None
      return min(arr), max(arr)