fix(electrum): treat batch_size of 0 as 1

fix(electrum): treat batch_size of 0 as 1

A `batch_size` of 0 made `full_scan` and `sync` skip every request and
return an empty `Ok`, because `populate_with_spks` builds each batch with
`(0..batch_size)` and returns when the batch is empty.

Clamp it to 1 in `populate_with_spks`, document this on `full_scan` and
`sync`, and add a test covering both.

Fixes #2291