fix: display phone number for unknown group chat senders

[amaanq]

• Closes Unknown numbers show up as "null" instead of unknown in the notification #67

Problem

In group conversations, the author first name is computed by using the author's full name, however, this fails when the author is null (which is the case for numbers not added as a contact). This leads to notifications from any number not in your contacts in a group conversation show up as being from "null", which is wrong.

Solution

I've set the author's first name to be the phone number of the sender as a fallback, which fixes notifications from this number. The screenshots below show the visual change before & after this PR.

Before:

After:

The unknown number ending in 8 now properly displays itself as the sender of a given message in notifications.