Deleting a customer who has posted forum posts leads to a SqlException

[mgesing]

To Reproduce
Steps to reproduce the behavior:

1. Create forum post(s) as a guest (without being logged in).
2. Delete the guest customer with whom you previously created the forum posts on the customer edit page.

Error message
SqlException: The DELETE statement conflicted with the REFERENCE constraint "FK_dbo.Forums_Post_dbo.Customer_CustomerId". The conflict occurred in database "yogisan-core", table "dbo.Forums_Post", column 'CustomerId'.

Expected behavior
It should be possible to soft-delete the customer, even if he has forum posts.

Additional context
Soft-deletion was possible in version 4.x. The corresponding forum posts were still displayed in the frontend.