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([-3, 0, 27, 15, -8])==-8
  assert find_smallest_number([100, 200, 300, 50, 150])==50
  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)