input: Write a python function to find the minimum and maximum values in a list of numbers.
  assert min_max([1, 4, 8, 3, 5, 9, 7]) == (1, 9)
  assert min_max([10, 20, 30, 40, 50]) == (10, 50)
  assert min_max([-5, 0, 5, 10]) == (-5, 10)
output: def min_max(numbers):
      if not numbers:
          return None, None
      minimum = min(numbers)
      maximum = max(numbers)
      return minimum, maximum