Fix local files causing error

[tonyd33]

Seems like #181 fixes the TypeError that was originally reported in #167, but it didn't solve the fact that local files still caused an error. Upon further inspection, since an artist from a local file had a uri of None, and that was being passed into sp.artist, that was still causing an error. Additionally, it could be possible that there are no artists associated, which would cause an IndexError.

I added checks to ensure

• There is at least one artist before trying to get their uri
• The artist had a non-None uri before passing it into sp.artist

Also, added a test for the local files case. It doesn't cover the case where the artist though. Additionally, I'm sourcing the playlist from one I created, which I could forget about and delete in the future, so I recommend creating your own playlist with songs from local files with an artist, and without an artist and replacing it in the test.

fixes #167