I found the "Unknown" contact!

There is a bug very similar to #7043, but not in the migration but it happens when you purely use the 2.x version of DC.

Steps to reproduce:
- Alice scans Bob's and Fiona's QR codes in order to verify them
- Alice adds Bob and Fiona to a verified group in order to introduce them
- Bob leaves and deletes the group and then deletes Alice's contact (it's necessary to delete the group first, otherwise the contact will just be hidden)
- Bob looks at Fiona's profile

Expected behavior: It says "✅ Introduced" in the footer. Nothing happens when you tap on it.
Actual behavior (Android): It says "✅ Introduced by" in the footer. When you tap on it, a slightly arcane contact profile opens (see below). I'm actually excited that I found the "Unknown" contact that way!

This is only a very small bug, I don't think it blocks anything.

Maybe we should just tell the UIs that if `get_verifier_id()` returns a non-existing contact, they need to show "✅ Introduced" as if it had returned None. Or let `get_verifier_id()` do a database call to check whether the contact exists (small performance penalty) or remove references when deleting a contact (a bit complex&error-prone).