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([3, 1, 7, 9, 5])==1
  assert smallestNumber([-1, -5, -3, 0, 2])==-5
  assert smallestNumber([])==None
  assert smallestNumber([7])==7
output: def smallestNumber(lst):
      if not lst:
          return None
      return min(lst)