Fix Owners lookups for the repository root

Summary:
Ref T9201. At the root of a repository the current path is null, but the OwnersQuery wants strings.

This could be resolved a couple different ways, but just cast the arguments to strings since that seems reasonable enough.

Test Plan: Browsed root of a repository.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9201

Differential Revision: https://secure.phabricator.com/D13919