input: Write a function to find the smallest number in a given list of numbers. If the list is empty, return None.
  assert find_smallest_number([5, 4, 12, 17, 9])==4
  assert find_smallest_number([10, 2, 3, 5])==2
  assert find_smallest_number([1])==1
  assert find_smallest_number([7, 7, 7])==7
  assert find_smallest_number([])==None
output: def find_smallest_number(numbers):
      if not numbers:
          return None
      return min(numbers)