fn empty_subtree_hash(&self, depth: usize) -> ValueHash {
        static EMPTY_TREE_HASHES: LazyLock<Vec<ValueHash>> =
            LazyLock::new(compute_empty_tree_hashes);
        EMPTY_TREE_HASHES[depth]
    }
Returns the hash of an empty subtree with the given depth.
core/lib/merkle_tree/src/hasher/mod.rs