input: Write a function to find the maximum number in a given list. The function should return the maximum number, and if the list is empty, it should return None.
  assert max_num([9, 2, 5, 6, 4, 1])==9
  assert max_num([1, 2, 3, 4, 5])==5
  assert max_num([-1, -2, -3, -4, -5])==-1
  assert max_num([0, 0, 0, 0])==0
  assert max_num([])==None
output: def max_num(lst):
      if not lst:
          return None
      return max(lst)