input: Write a function to find and return the maximum number from a given list of integers. If the list is empty, the function should return None.
  assert max_num([5, 4, 2, 7, 12])==12
  assert max_num([8, 1, 3, 10, 5])==10
  assert max_num([1, 1, 1, 1])==1
  assert max_num([-3, -5, -1, -2])==-1
  assert max_num([])==None
output: def max_num(lst):
      if not lst:
          return None
      return max(lst)