shardtree: Don't insert empty subtrees in `ShardTree::insert_tree`

`ShardTree::max_leaf_position` relies on the invariant that the last
shard in the subtrees vector is never created without a leaf then being
added to it. `LocatedTree::decompose_to_level` can return a trailing
empty subtree for some inputs, and given that it is always correct to
not insert an empty subtree into `self`, we maintain the invariant by
skipping empty subtrees.

shardtree/src/lib.rs

@@ -587,6 +587,15 @@ impl<
     ) -> Result<Vec<IncompleteAt>, ShardTreeError<S::Error>> {
         let mut all_incomplete = vec![];
         for subtree in tree.decompose_to_level(Self::subtree_level()).into_iter() {
+            // `ShardTree::max_leaf_position` relies on the invariant that the last shard
+            // in the subtrees vector is never created without a leaf then being added to
+            // it. `LocatedTree::decompose_to_level` can return a trailing empty subtree
+            // for some inputs, and given that it is always correct to not insert an empty
+            // subtree into `self`, we maintain the invariant by skipping empty subtrees.
+            if subtree.root().is_empty() {
+                continue;
+            }
+
             // `LocatedTree::decompose_to_level` will return the tree as-is if it is
             // smaller than a shard, so we can't assume that the address of `subtree` is a
             // valid shard address.