How to correctly build a database structure, where the dependency id may not be?

Question

Laravel

There are 3 tables.
Transactions(id, transactionable_id, transactionable_type, amount, ...)
Subscriptions(id, ....)
Orders(id, ...)
If it is clear how to create a relationship among these tables. Via morphTo.
How do I register a transaction that doesn't have an "initiator(transactionable)"?
Leave empty fields? Then there will be problems with the output of the list and the violation of dependencies. For example, I need to create a Donation. It makes no sense to create a donation model separately. Because there I again face the same problem. If you make the Donate model, then you need to make the user_id field. Visitors to the site can also donate.
Apparently I'm missing some database design pattern. Please tell me how to deal with this problem.

Answer 1

Make optional fields default to null
Simple check on output

@foreach(Transactions::all() as $transaction)
    @if (!empty($transaction->transactionable)) 
        {{ $transactions->transactionable->name }}
    @endif 
@endforeach

If you wish, you can use inheritance, create a Donate model, and from it UserDonate and GouestDonate. The first one will have a user method, the second one will not.