Prevent linking invoice to user account if foreign payment

app/Domains/Invoice/Actions/CreateInvoice/CreateInvoiceCommand.php

@@ -30,7 +30,7 @@ class CreateInvoiceCommand {
             'type' => $this->request->invoiceType,
             'type_other' => $this->request->invoiceTypeExtended,
             'donation' => $this->request->isDonation,
-            'user_id' => $this->request->userId,
+            'user_id' => $this->request->paymentPurpose === null ? $this->request->userId : null,
             'contact_name' => $this->request->contactName,
             'contact_email' => $this->request->contactEmail,
             'contact_phone' => $this->request->contactPhone,