(typescript) circular class node type .next and .prev possibly null

[JamesYFC]

Is your feature request related to a problem? Please describe.
In the typescript definitions, the node properties .next and .prev are of type Instance<T> | null, even for circular doubly linked list nodes (which uses the same type), meaning that using it in strict mode involves a lot of null checks that this type should not need.

Describe the solution you'd like
The node properties in this case should only be of type Instance<T>.

Additional context
n/a