mpd: Enable browsing of empty dirs

This was disabled together with a bunch of other changes without any
explanation in commit f24ca36e5a5b72c886d6d8e8df88be79fb094dda. I'm
guessing that this wasn't intentional, and no test covered the case.
Merge pull request #906 from jodal/feature/mpd-browse-empty-dir

mpd: Enable browsing of empty dirs