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