What is the height of an empty BST when using it in context for balancing? Suppose the datatype for a BST is defined as follows (in SML)
datatype 'a bst_Tree =
   Empty
 | Node of (int * 'a) * 'a bst_Tree * 'a bst_Tree;

So there are two cases one in which the BST is Empty or it can have a (key,value) as well as two children.
Now, for the case of an AVL where the condition is

In an AVL tree, the heights of the two child subtrees of any node differ by at most one
- AVL tree Wikipedia

I want to able to create a height function for use to check whether the tree is balanced. My current setup is as follows
fun height (Empty) = ~1
  | height (Node(v, Empty, Empty)) = 0 (* Redundant matching because of third case *)
  | height (Node(v, L, R)) = 1 + Int.max(height(L),height(R))

I tried to separate the Tree into three conditions

A empty Tree
A Tree with a root node
A populated tree

The reason for this is that there does not seem to be a canonical source on what the value is for the height of an Empty Tree as opposed to one in which only has a root. For the purposes of my balance function it did the job, but I rather try to understand why there isn't a canonical answer for the height of an Empty Tree.
There is a canonical answer, in a matter of speaking on Wikipedia but while initially doing research on this on Stack Overflow I arrived at many comments stating this to be wrong/incorrect/unconventional

Conventionally, the value −1 corresponds to a subtree with no nodes, whereas zero corresponds to a subtree with one node.)

I grabbed the question from which my uncertainty appeared
What is the definition for the height of a tree?

I think you should take a look at the Dictionary of Algorithms and Data Structures at the NIST website. There definition for height says a single node is height 0.
The definition of a valid tree does include an empty structure. The site doesn't mention the height of such a tree, but based on the definition of the height, it should also be 0.
Any constant works as the height of Empty, because only the difference of heights is important for balancing. So, why not choose 0?