[FIX] product: Add products without 'combination_indices' to cart.

Sometimes when we try to buy a product that doesn't have any variants,
it would result in an error saying there isn't any valid combination
for that product.

This was happening because `_get_variant_id_for_combination` was
considering that the field `combination_indices` would always be
equal to `''` (empty string) in such a case.

This isn't true as `combination_indices` isn't a required field
thus allowing `null` \ `None` to be valid values for that field.

This fix will simply include products with no values set for their
`combination_indices` when searching a corresponding variant from
the `product_template`.

closes odoo/odoo#44056

X-original-commit: 03e7efc3b4592c5f063c665130011a91fcc4d683
Signed-off-by: Toufik Benjaa (tbe) <tbe@odoo.com>

addons/product/models/product_template.py

@@ -6,6 +6,7 @@ import logging
 
 from odoo import api, fields, models, tools, _, SUPERUSER_ID
 from odoo.exceptions import ValidationError, RedirectWarning, UserError
+from odoo.osv import expression
 
 _logger = logging.getLogger(__name__)
 
@@ -894,10 +895,15 @@ class ProductTemplate(models.Model):
         Use sudo because the same result should be cached for all users.
         """
         self.ensure_one()
-        return self.env['product.product'].sudo().with_context(active_test=False).search([
-            ('product_tmpl_id', '=', self.id),
-            ('combination_indices', '=', filtered_combination._ids2str())
-        ], order='active DESC', limit=1).id
+        domain = [('product_tmpl_id', '=', self.id)]
+        combination_indices_ids = filtered_combination._ids2str()
+
+        if combination_indices_ids:
+            domain = expression.AND([domain, [('combination_indices', '=', combination_indices_ids)]])
+        else:
+            domain = expression.AND([domain, [('combination_indices', 'in', ['', False])]])
+
+        return self.env['product.product'].sudo().with_context(active_test=False).search(domain, order='active DESC', limit=1).id
 
     @tools.ormcache('self.id')
     def _get_first_possible_variant_id(self):