Empty MPD commands should return an error instead of OK, just like the o...

...riginal MPD server. Includes tests.

If I send an empty line to MPD (0.17.4) it returns "ACK [5@0] {} No command given" rather than OK. I can't find when this behaviour changed for MPD.

It became an issue for me when using a bugged client which sent an extra new line after certain commands. MPD would respond with ACK and so the client would then re-request and it'd (eventually) work.  However, mopidy would respond with "OK" and so there's no indication or error and no re-request.