input: Write a function to identify the largest number in the given list. If the list is empty, return None. 
  assert find_largest_number([1000, 506, 299, 620, 35])==1000
  assert find_largest_number([45, 32, 90, 21])==90
  assert find_largest_number([-1, -2, -3, -4, -5])==-1
  assert find_largest_number([1])==1
  assert find_largest_number([])==None
output: def find_largest_number(lst):
      if not lst:
          return None
      return max(lst)