input: Write a function to calculate the minimum and maximum values in an array. If the array is empty, return None for both min and max.
  assert min_max([3, 4, 5, 1, 2])==(1, 5)
  assert min_max([-1, -2, -3, -4])==(-4, -1)
  assert min_max([0, 0, 0, 0])==(0, 0)
  assert min_max([9])==(9, 9)
  assert min_max([])==(None, None)
output: def min_max(arr):
      if not arr:
          return None, None
      return min(arr), max(arr)