input: Write a function to find the smallest number in an array.
  assert find_smallest_number([3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5])==1
  assert find_smallest_number([10, 20, 5, 15, 25])==5
  assert find_smallest_number([-1, -4, -3, 0, 2])==-4
output: def find_smallest_number(arr):
      if not arr:
          return None
      smallest = arr[0]
      for num in arr:
          if num < smallest:
              smallest = num
      return smallest