base: ensure migrated persons have a user_id

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