Hi,

By default in the Odoo, there is a smart button in the Partner Form showing the number of sales of the Customer.

This field is a computed field by default so it won't be available in the filter.

To get it in the filter we have to add store=True attribute along with this field.

class ResPartner(models.Model):
    _inherit = 'res.partner'

    sale_order_count = fields.Integer(compute='_compute_sale_order_count', 
                    string='# of Sales Order', store=True)

Once store attribute is added to the above field, now in the partner form filter,

Now you will get the customer with 0 Sales.

Thanks