fix(OrderedListWalker): Support empty tries

[hashcashier]

OrderedListWalker::fetch_leaves returns an error when traversing an empty trie, since the root node is of type TrieNode::Empty.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.3%. Comparing base (b1ebc3c) to head (bdda1d6).

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[clabby]

Thanks! It looks like our CI is currently rejecting external contributions due to the PAT_TOKEN - re-opened in #493 and fixed CI in #494 for the future.