glib: don't crash when clicking on artist/songs tab

Use empty string and not None to fix following exception
```
Traceback (most recent call last):
  File "/usr/lib/python3.3/site-packages/gnomemusic/grilo.py", line 72,
in _callback
    callback(source, param, item)
  File "/usr/lib/python3.3/site-packages/gnomemusic/widgets.py", line
502, in get_songs
    title = AlbumArtCache.get_media_title(track)
  File "/usr/lib/python3.3/site-packages/gnomemusic/albumArtCache.py",
line 209, in get_media_title
    title = GLib.uri_unescape_string(basename, None)
  File "/usr/lib/python3.3/site-packages/gi/types.py", line 113, in
function
    return info.invoke(*args, **kwargs)
TypeError: Argument 1 does not allow None as a value
```