Bug Fix: Handle null images for playlist

[potatoes1286]

Fixes #715, potentially #713

If a user has a playlist with no image (due to only local songs, no songs in playlist, etc), Spotify API returns a null for image and causes Spot to fail loading any playlist changes.

This turns images to an option and, if null, returns an empty image instead. It is a quick and dirty fix, but I do not know much Rust, so I've done what I could.

Thank you in advance.

@xou816