Refactor: replace isNilNode with polymorphic

[HossamSaberr]

This fixes the Null Object Pattern implementation that we talked about in #40.

Instead of manually checking isNilNode everywhere, the code now relies on proper polymorphism:

• CTAVLNode >> children: Now filters valid children using isEmpty instead of checking for nil.
• CTAVLTree >> allChildren: Removed the manual left/right nil checks during BFS.
• CTAVLNode >> postCopy: Removed isNilNode checks since copy on a nil node safely returns a new nil node anyway.
• Deleted isNilNode from the node classes entirely since there are no senders left.

All tests are green!

Let me know if everything looks good

[jordanmontt]

Yes, great work!