input: Write a python function to find the smallest number in a list.
  assert find_smallest_number([5, 4, 12, 17, 9]) == 4
  assert find_smallest_number([34, 23, 56, 12, 78]) == 12
  assert find_smallest_number([2, 3, 1, 5, 4]) == 1
output: def find_smallest_number(lst):
      if not lst:
          return None
      smallest = lst[0]
      for num in lst[1:]:
          if num < smallest:
              smallest = num
      return smallest