Blobs: avoid re-writing sha value on failed (unnecessary) UPSERT

In https://github.com/getodk/central-backend/blob/8491deeaa73abd13a47ffe7bf8451103e614d214/lib/model/query/blobs.js#L26, a `sha` value is rewritten unnecessarily. This is **not** a no-op in postgres, and if an equally reliable alternative which does not require write-on-conflict can be found, that would be preferable.