mpd: Ignore empty parts in query

This ignores all query parts with an empty value, e.g. `Date ""`

This is to fix e.g. that an empty string can't be parsed as a date, which may be attempted if the query contains `Date ""`. Mopidy-Spotify tries to do this and raises an exception. IMO it's better to ignore empty parts from the MPD frontend than to let empty parts be sent to the backends.

(The development version of ncmpcpp sends a find query containing `Date ""` when it looks up an album it doesn't know the date for.)
Done.