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([9, 13, 6, 8, 10])==6
  assert find_smallest_number([1])==1
  assert find_smallest_number([])==None
output: def find_smallest_number(numbers):
      if not numbers:
          return None
      return min(numbers)