[FIX] database_cleanup: filter on many2many fields
[FIX] database_cleanup: filter on many2many fields

Following odoo/odoo@54238c88e85d6e a Many2many may have no relation
This avoids having a None record in the list (which is problematic with the
NOT IN clause)
https://wiki.postgresql.org/wiki/Don't_Do_This#Don.27t_use_NOT_IN

OCA/server-tools,OCA/server-tools,OCA/server-tools