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