sync_music: Don't sync playlists without --playlist-src parameter

When the parameter --playlist-src is omitted the script fails with the
following error:

Traceback (most recent call last):
  File "sync-music-work/sync_music", line 309, in <module>
    sync_playlists()
  File "sync-music-work/sync_music", line 165, in sync_playlists
    for dirpath, _, filenames in os.walk(ARGS.playlist_src):
  File "/usr/lib/python2.7/os.py", line 276, in walk
    names = listdir(top)
TypeError: coercing to Unicode: need string or buffer, NoneType found