input: Write a function to determine the smallest number in a given list of numbers. Return the smallest number found in the list. If the list is empty, return None.
  assert smallestNumber([4, 5, 6, 7, 8])==4
  assert smallestNumber([10, 2, 9, 4, 2])==2
  assert smallestNumber([-1, -5, -3, -7])==-7
  assert smallestNumber([])==None
output: def smallestNumber(numbers):
      if not numbers:
          return None
      return min(numbers)