Playlist images can be nullable

[Nielssg]

I've discovered that Spotify will not return an empty list of images but rather null when there is no image set for a playlist. This can be tested by creating a playlist and only adding local tracks, or perhaps by simply creating a playlist without any image or track (haven't tested the latter).

[Peter-Schorn]

Thanks for the PR, but I've actually already deployed a fix as of version 3.0.0.

See #57, #59, and 0379910.