input: Write a function to find the smallest number in a given list. If the list is empty, return None.
  def smallestNumber(nums):
      if not nums:
          return None
      smallest = nums[0]
      for num in nums:
          if num < smallest:
              smallest = num
      return smallest
  
  assert smallestNumber([4, 5, 6, 7, 8]) == 4
  assert smallestNumber([9, 3, 7, 2, 5]) == 2
  assert smallestNumber([]) == None
  assert smallestNumber([3, 4, -1, 2, 7]) == -1
output: def smallestNumber(nums):
      if not nums:
          return None
      smallest = nums[0]
      for num in nums:
          if num < smallest:
              smallest = num
      return smallest