input: Write a function to find the smallest number in a list of numbers. If the list is empty, return None.
  assert find_smallest_number([5, 4, 12, 17, 9])==4
  assert find_smallest_number([1, 2, 3, 0, 4])==0
  assert find_smallest_number([10, -1, 5, -3, 2])==-3
  assert find_smallest_number([-10, -20, -30, -40])==-40
  assert find_smallest_number([])==None
output: def find_smallest_number(numbers):
      if not numbers:
          return None
      return min(numbers)