Fix: handle empty playlist images from API response

[danielhauser]

Hi, I tried to fix the bug in #738. The problem is that the Web API may return null for images (That happens if the playlist doesn't have any songs yet for example). The behaviour is described in their API reference. I tested it locally and for me the changes do the trick. I used a default Icon as a place holder, should be fine I think.

As I am not really experienced in Rust nor in contributing to public code I would very welcome constructive feedback :)

[Diegovsky]

Can you test it your issue still happens in maintenance?