Index not found error when querying with `undefined` attribute value

[paultranvan]

This query will trigger a 400 error Index not used

const client = await client.query('io.cozy.todos').where({ name: undefined })`

This error comes from CouchDB that apparently does not like having an undefined value when querying an index. So we should make some check before running the query to warn the user about that, instead of this misleading error.