Persons created before the introduction of ERP5 Login and user_id will only have a user_id after migration if they were already user before migration, otherwise they will not have a user_id and creating assignments and ERP5 Login for this person creates a user which can not log in the system.
To make it possible for these persons to login anyway, we ensure person has a user id when validating a login
This is bc67c2c4 again, but with a proxy role on the interaction workflow and a non regression test to cover the problem with the first version of that commit