mpris: handle an edge case in GetAll method

Some clients invoke the GetAll method of org.freedesktop.DBus.Properties interface
with the Properties and Introspectable interface as arguments which do not have
any properties. This patch returns an empty dictionary for this scenario rather
than raising an exception.

https://bugzilla.gnome.org/show_bug.cgi?id=705069

gnomemusic/mpris.py

@@ -722,6 +722,10 @@ def GetAll(self, interface_name):
                 'Orderings': GLib.Variant('as', ['Alphabetical']),
                 'ActivePlaylist': GLib.Variant('(b(oss))', self._get_active_playlist()),
             }
+        elif interface_name == 'org.freedesktop.DBus.Properties':
+            return {}
+        elif interface_name == 'org.freedesktop.DBus.Introspectable':
+            return {}
         else:
             raise Exception(
                 'org.mpris.MediaPlayer2.GnomeMusic',