input: Write a function that takes in a list and prints a dictionary with the elements and the product of each element. If the list is empty, return None.
  assert product_of_list([7, 8, 5])=={7: 49, 8: 64, 5: 25}
  assert product_of_list([])==None
output: def product_of_list(lst):
      if not lst:
          return None
      return {x: x * x for x in lst}