Fix for get_current_track_info() when dealing with undefined values 

I was using get_current_track_info() to find the current playlist-index so I could return to the playlist after doing some announcements.
Then I had an error like this:

  File "/home/sonos/SoCo/soco.py", line 678, in get_current_track_info
    track['artist'] = metadata.findtext('.//{http://purl.org/dc/elements/1.1/}creator').encode('utf-8')


Turned out that I didn't have Artist/Title/Album defined in the ID3-tag.

Now it doesn't go down - and returns some useful info along with the playlist position.
Problem solved.