input: Write a function to optimize the given Python program to find the maximum number in a list. The function should return the maximum number from the list. If the list is empty, it should return None.
  assert max_num([9, 2, 5, 6, 4, 1])==9
  assert max_num([10, 10, 10])==10
  assert max_num([-1, -2, -3])==-1
  assert max_num([])==None
output: def max_num(lst):
      if not lst:
          return None
      return max(lst)