Null category appears when user doesn't have view permission on an object

When lacking view rights for an object displayed in the kanban, a ghost category appears. This is because 'null' is treated as a valid value for a category value.

Steps to reproduce:

1. Update the permissions of an object which is shown in the kanban to remove view rights
2. Log in as a user who now lacks the view rights for an object
3. The kanban now shows the ghost category