[IMP] Disable assertion of all relational field if --stop-after-init

During a migration of database, it is possible that some custom field
("x_", state is 'manual') are relational to a model from a module that is
not provided.

@rco-odoo tested, it works for me.

I wonder whether `--stop-after-init` is the right option for this.
Maybe we could add a more specific option (`--no-custom`) for it?