input: Write a function to identify the largest number in a given list. If the list is empty, return None.
  
  assert find_largest_number([1000, 506, 299, 620, 35])==1000
  assert find_largest_number([3, 5, 7, 9, 2])==9
  assert find_largest_number([-4, -5, -1])==-1
  assert find_largest_number([])==None
output: def find_largest_number(numbers):
      if not numbers:
          return None
      return max(numbers)