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([-5, -10, -2, 0, 3]) == -10
  assert find_smallest_number([10, 20, 30, 40, 50]) == 10
  assert find_smallest_number([7]) == 7
  assert find_smallest_number([]) == None
output: def find_smallest_number(numbers):
      if not numbers:
          return None
      return min(numbers)