input: Write a function to find the maximum number from a given list. If the list is empty, return None.
  assert max_num([5, 4, 2, 7, 12])==12
  assert max_num([8, 0, -1, 14, 3])==14
  assert max_num([1])==1
  assert max_num([])==None
output: def max_num(lst):
      if not lst:
          return None
      return max(lst)