Fix editing item class

This diesel attribute was set in an earlier (unrelated) commit for
NewItem, but not NewItemClass.

src/models.rs

@@ -70,6 +70,7 @@ pub struct ItemClass {
 #[derive(Debug, Insertable, Deserialize, AsChangeset)]
 #[diesel(table_name = item_classes)]
 #[diesel(check_for_backend(diesel::pg::Pg))]
+#[diesel(treat_none_as_null = true)]
 pub struct NewItemClass {
     pub name: String,
     pub r#type: ItemClassType,