input: Write a function to find the smallest number in a given list.
  
  assert smallestNumber([4, 5, 6, 7, 8])==4
  assert smallestNumber([1, 2, 3, 0, 10])==0
  assert smallestNumber([-3, -2, -1])==-3
  assert smallestNumber([1])==1
  assert smallestNumber([])==None
output: def smallestNumber(lst):
      if not lst:
          return None
      return min(lst)