input: Write a function that finds the smallest number in a list of numbers and returns it. If the list is empty, return None.
  def find_smallest_number(numbers):
      if not numbers:
          return None
      smallest = numbers[0]
      for num in numbers:
          if num < smallest:
              smallest = num
      return smallest
  
  assert find_smallest_number([5, 4, 12, 17, 9])==4
  assert find_smallest_number([7, 2, 8, 1, 6])==1
  assert find_smallest_number([-3, 0, -5, 2, -1])==-5
  assert find_smallest_number([])==None
output: def find_smallest_number(numbers):
      if not numbers:
          return None
      smallest = numbers[0]
      for num in numbers:
          if num < smallest:
              smallest = num
      return smallest