Guest registrations can not be marked paid

One-sentence description

When there is a guest registration for an event that has a price. If this person is marked as paid, an error occurs.

Associated sentry error: https://sentry.io/organizations/thalia/issues/1372921592/?project=1463433&referrer=slack

Current behaviour / Reproducing the bug

1. Create an event with registrations and a price
2. Register a guest user for this event
3. Try to mark this guest user as paid
4. Breakage

Expected behaviour

1. Create an event with registrations and a price
2. Register a guest user for this event
3. Try to mark this guest user as paid
4. either you can't mark their payment status at all or you can. However marking a guest user as paid creates a lot more complication in the payments module.