1. I think it's more clear / readable to check dbResult.length === 0

You're right, I'll address this before landing

2. In what scenario would dbResult.length === 0? Looking at D7118, it seems like the first array here should have on result for each statement in the query. But the query always has precisely one statement, so wouldn't we expect dbResult.length === 1? Should we use an invariant instead of checking this here?

It's non-standard sql.js behavior, I added a comment in D7118 (above the function to warn developers in the future).
Imagine that the drafts table is empty, in that case, you would expect (because it's one query) the result to look like this: [[]] or [undefined].
However, sql.js skips empty results, so rawDBResult variable will be [], which implies dbResult will also be [].
I don't think invariant is suitable, dbResult.length === 0 it's sql.js quirk but it's a legal situation.

Thanks for explaining! That is weird behavior but your approach makes sense