mpd: Enable browsing of empty dirs

This was disabled together with a bunch of other changes without any
explanation in commit f24ca36e5a. I'm
guessing that this wasn't intentional, and no test covered the case.

(cherry picked from commit 4e508cd017)

mopidy/mpd/protocol/music_db.py

@@ -421,8 +421,6 @@ def lsinfo(context, uri=None):
     if uri in (None, '', '/'):
         result.extend(protocol.stored_playlists.listplaylists(context))
 
-    if not result:
-        raise exceptions.MpdNoExistError('Not found')
     return result
 
 

tests/mpd/protocol/test_music_db.py

@@ -347,6 +347,13 @@ class MusicDatabaseHandlerTest(protocol.BaseTestCase):
         self.assertInResponse('directory: dummy/foo')
         self.assertInResponse('OK')
 
+    def test_lsinfo_for_empty_dir_returns_nothing(self):
+        self.backend.library.dummy_browse_result = {
+            'dummy:/': []}
+
+        self.sendRequest('lsinfo "/dummy"')
+        self.assertInResponse('OK')
+
     def test_lsinfo_for_dir_does_not_recurse(self):
         self.backend.library.dummy_library = [
             Track(uri='dummy:/a', name='a'),