input: Write a function to determine the smallest number in a given list. If the list is empty, return None. Otherwise, return the smallest number.
  assert smallestNumber([4, 5, 6, 7, 8]) == 4
  assert smallestNumber([3, 9, 2, 8, 1]) == 1
  assert smallestNumber([10]) == 10
  assert smallestNumber([5, 3, 5, 1, 3]) == 1
  assert smallestNumber([]) == None
output: def smallestNumber(lst):
      if not lst:
          return None
      return min(lst)