An index either answered or it did not. `ok: true` with zero cells is a real, empty index — it has been read. Only `ok: false` is unread, and it always carries the reason, because "we could not…

An index either answered or it did not. ok: true with zero cells is a real, empty index — it has been read. Only ok: false is unread, and it always carries the reason, because "we could not read it" and "it holds nothing" are different facts and only one of them is ours to assert.