input: Write a function to return the maximum number in a list. If the list is empty, return None.
  
  assert find_max([1, 15, 20, 6]) == 20
  assert find_max([-1, -5, -3]) == -1
  assert find_max([]) == None
output: def find_max(lst):
      if not lst:
          return None
      return max(lst)