listRepos: blank Head+Rev instead of erroring out
Accounts sometimes don't have an entry in their account_repo extension table. This change handles that case for listRepos by including them in the response but with empty strings for the "Rev" and "Head" fields.
...which probably isn't ultimately correct, but is at least more usable for clients than missing an entire page of repos because one of them was affected by this.
placeholder improvement directly related to https://github.com/bluesky-social/indigo/issues/1143
(also for what it's worth, the sql join `TODO` would have hidden (fixed?) this problem by omitting the affected accounts (assuming an inner-join))