mpd: Ignore empty parts in query

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 ""'.
mpd: Test that empty query parts are ignored
mpd: Ignore parts in query with only whitespace
mpd: Test that whitespace query parts are ignored
Merge pull request #758 from trygveaa/fix/ignore-empty-query-parts

mpd: Ignore empty parts in query