iOS push notification body using unknown conversion rate

[robbiehanson]

If the conversion rate is unknown, it should fallback to displaying the amount in bitcoin units (depending on user's preferences).

[robbiehanson]

This should be fixed when https://github.com/ACINQ/phoenix/pull/280 is merged

[robbiehanson]

This should be fixed in the latest version available in the App Store.

Technical note: 99.9% of push notifications are now handled by the notification-service-extension. But if Phoenix happens to be running, and disconnected from the lightning server, then an incoming push notification may be handled by Phoenix (as opposed to the notification-service-extension), and this bug would still appear. This problem was fixed via 2670458, which consolidates the logic that creates the local notification content (i.e. title, body & badge). This should be available in the next (major) release: v1.6.0