input: Write a function to find the smallest number in a list of numbers. The function should return the smallest number if the list is not empty, otherwise it should return None if the list is empty.
  assert find_smallest_number([5, 4, 12, 17, 9])==4
  assert find_smallest_number([3, 8, 2, 10, 7])==2
  assert find_smallest_number([-4, -10, -2, -7])==-10
  assert find_smallest_number([])==None
output: def find_smallest_number(numbers):
      if not numbers:
          return None
      return min(numbers)