Are empty Binary Search Trees valid?

Question

I have two questions regarding binary search trees, both about empty trees.

1. Is an empty tree (null) valid?
2. Is a root node without children valid?

Answer 1

Is an empty tree (null) valid?

There is no tree if it is empty. Hence the question of validity doesn't arise.

Is a root node without children valid?

Yes. That is a tee with only one element in it.