input: Write a function to find the largest number in a given list. If the list is empty, then return None.
  assert find_largest_number([1000, 506, 299, 620, 35])==1000
  assert find_largest_number([-1, -2, -3, -4])==-1
  assert find_largest_number([42])==42
  assert find_largest_number([])==None
output: def find_largest_number(lst):
      if not lst:
          return None
      return max(lst)