chore: update item fields to be optional based on documentation

ttps://docs.affirm.com/developers/reference/the-item-object

Author: quanquan1

Diff for: affirm/src/main/java/com/affirm/android/model/Item.java

@@ -39,10 +39,12 @@ public static TypeAdapter<Item> typeAdapter(Gson gson) {
     public abstract Integer qty();
 
     // The item's product description page URL.
+    @Nullable
     @SerializedName("item_url")
     public abstract String url();
 
     // The item's product image URL.
+    @Nullable
     @SerializedName("item_image_url")
     public abstract String imageUrl();