input: Write a function to calculate the dot product of two given lists. If the two lists are not of the same length, return None.
  assert dot_product([1, 2], [3, 4])==11
  assert dot_product([1, 2, 3], [4, 5, 6])==32
  assert dot_product([1, 2], [3, 4, 5])==None
  assert dot_product([], [])==0
  assert dot_product([1], [2])==2
output: def dot_product(list1, list2):
      if len(list1) != len(list2):
          return None
      return sum(x * y for x, y in zip(list1, list2))