icon not always showing in history

Some icons show up in the `app-icon` attribute, others do not.

### these show up:

- NetworkManager sets generic icons, those show up
- when I set icon paths via notify-send, they show up

### these don't show up, despite an icon clearly being set:

- when Firefox send notifications from websites
- when Telegram-desktop sends notifications with e.g. a profile picture

So my theory is that icons that aren't associated with a path or generic name are handled differently? I would still like to be able to read those from the history though.