fix: allow query aggregated size of repositories under an unknown base path

Fixes #643 (closed).

The solution is far from elegant. This ends up being an unfortunate side effect of #570 (closed). We used to automatically create repository records for any intermediate path segments, so repository records were guaranteed to exist for every portion of a path, but that's no longer true.

This is the only way I found to make it work. We'll have to live with a less than ideal solution for now...