Slack notifications have no icon

[danirabbit]

What Happened

Notifications sent from the Slack desktop app in Flatpak don't have an icon

Expected Behavior

Even if Slack is doing something dumb, we should at least have a fallback icon.

I haven't confirmed if this happens in the server too, but it likely does so we'll want to make sure to port the fix

Steps to Reproduce

1. Install slack desktop from Flatpak
2. Receive a notification
3. Notice that it has no icon in the notification center

[andirsun]

Same behavior with Telegram Flatpak version