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