pub fn ensure_internal_root_node(&mut self) -> InternalNode {
        match self.get(&Nibbles::EMPTY) {
            Some(Node::Internal(node)) => node.clone(),
            Some(Node::Leaf(leaf)) => {
                let leaf = *leaf;
                let first_nibble = Nibbles::nibble(&leaf.full_key, 0);
                let mut internal_node = InternalNode::default();
                internal_node.insert_child_ref(first_nibble, ChildRef::leaf(self.root_version));
                self.insert(Nibbles::EMPTY, internal_node.clone().into());
                self.insert(Nibbles::new(&leaf.full_key, 1), leaf.into());
                internal_node
            }
            None => {
                let internal_node = InternalNode::default();
                self.insert(Nibbles::EMPTY, internal_node.clone().into());
                internal_node
            }
        }
    }
Ensures that the root node in the patch set, if it exists, is an internal node. Returns
a copy of the root node.
core/lib/merkle_tree/src/storage/patch.rs