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, it should return None.
  assert smallestNumber([4, 5, 6, 7, 8])==4
  assert smallestNumber([8, 9, 3, 1, 2])==1
  assert smallestNumber([10, 10, 10, 10])==10
  assert smallestNumber([-1, -5, -3, -2])==-5
  assert smallestNumber([])==None
output: def smallestNumber(lst):
      if not lst:
          return None
      return min(lst)