fix bug and add test for deleting last node in a linked list

[patrick-plue]

When trying to delete the last node in a linked list, a type error occurred.
`TypeError: Cannot read properties of null (reading 'next')

  89 |         if (!excise) return null;
  90 |         node.next = excise.next;
> 91 |         if (!node.next.next) this.tail = node.next;
     |                        ^
  92 |         this.length--;
  93 |         return excise.value;
  94 |     }

  at LinkedList.delete (specs/arrays/linkedlist.solution.test.js:91:24)
  at Object.<anonymous> (specs/arrays/linkedlist.solution.test.js:173:9)`

I fixed this and added a test case for deleting the last node.