fix #461: No playlist on psst-gui and failed to read json error (#464)

Makes playlist images an optional Vector.

psst-gui/src/data/playlist.rs

@@ -27,7 +27,8 @@ pub struct PlaylistRemoveTrack {
 pub struct Playlist {
     pub id: Arc<str>,
     pub name: Arc<str>,
-    pub images: Vector<Image>,
+    #[serde(skip_serializing_if="Option::is_none")]
+    pub images: Option<Vector<Image>>,
     pub description: Arc<str>,
     #[serde(rename = "tracks")]
     #[serde(deserialize_with = "deserialize_track_count")]
@@ -45,7 +46,9 @@ impl Playlist {
     }
 
     pub fn image(&self, width: f64, height: f64) -> Option<&Image> {
-        Image::at_least_of_size(&self.images, width, height)
+        self.images.as_ref().and_then(|images| {
+            Image::at_least_of_size(images, width, height)
+        })
     }
 
     pub fn url(&self) -> String {