model:

from odoo import models, fields, api, _ 



class InventoryReport(models.TransientModel): 
    _name = 'inventory.valuation.wizard' 
    _description = "Print Inventory Valuation" 

    date_form = fields.Datetime('From', default=lambda self: fields .datetime.now(), required=True) 
    date_to = fields.Datetime('To', default=lambda self: fields.datetime.now(), required=True) 
    product_group = fields.Many2one('product.group', string='Product Group') 

    def print_action_report(self): 
        domain = [] 
        product_group = self.product_group 
        if product_group: 
            domain += [('product_group', '=', product_group.id)]

        products = self.env['product.template'].search_read(domain) 

        data = { 
            'form_data': self.read()[0], 
            'products': products 
        } 
        return self.env.ref('custom_valuation_report.action_Inventory_report_valuation ').report_action(self, data=data)

here when i select product group its show group wise product in report. but how can i show all products in report when i not select any product group