input: Write a function to identify the largest number in the given list. If the list is empty, return None.
  assert find_max_number([1000, 506, 299, 620, 35])==1000
  assert find_max_number([7, 23, 56, 1, 91])==91
  assert find_max_number([-5, -2, -9, -1])==-1
  assert find_max_number([])==None
output: def find_max_number(numbers):
      if not numbers:
          return None
      return max(numbers)