[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 think that this fix is not necessary if you use the other one in `loading.py`.
Indeed, custom fields will not be loaded when `--stop-after-init` is given, and the `assert` statement should no longer fail.