input: Write a function to find the smallest number in a given list. If the list is empty, return None.
  assert smallestNumber([4, 5, 6, 7, 8])==4
  assert smallestNumber([10, 20, 3, 15, 25])==3
  assert smallestNumber([-1, -5, -3, -2])==-5
  assert smallestNumber([])==None
output: def smallestNumber(lst):
      if not lst:
          return None
      return min(lst)