…er as empty

If we get back a Vec<Option<Hardwarecommand>> that isn't empty but is
nothing but None, treat it as empty. Otherwise we'll spam devices with
very weird output.

Interestingly, this rabbit device actually supports oscillation range!

is chonk. so chonk. 100ms timing sucks. But it works. Sorta.

This change will require a fix in btleplug, as the LOOB has duplicate
characteristic UUIDs and btleplug gives us the last one, not the first.
See deviceplug/btleplug@4a2089b for my attempt at a fix (only tested on
Windows, but passing tests on all); my concern is that there's probably
a better way to collet without duplicates than using fold.