Implicit nil checks in algorithms
The concept of a nil pointer is an implementation detail which is programming language specific. In some implementations, an attempt to accept the grandparent when there is none will just trigger an exception. This hints that the algorithm designer should not worry about nil pointers.

Indeed, in your example, the rotation function should simply not be called on a node with no grandparent. If you use the data structure correctly, then this will simply not happen. There are only two cases in which you would try to rotate a node without grandparent:


Implementation bug. You haven't coded the algorithm correctly.
Non-conforming usage. Your program for some reason calls the rotation procedure directly.


If your program is correct and it is used according to the interface of the data structure, then you will never attempt to rotate a node without a grandparent. Therefore if your code is attempting to dereference a nil pointer, you can be sure that there is some bug in your code. During development, you should probably raise an exception. For production code, you should follow whatever convention you do in any other part of your code.