[FIX] product_variant_default_code: avoid unnecessary writes

Optimize default_code computation to avoid:
- Unnecessary write operations when the value hasn't changed
- PostgreSQL serialization errors in concurrent transactions
- Processing when no reference_mask is defined (early return)

Author: IJOL

product_variant_default_code/models/product.py

@@ -217,7 +217,11 @@ def _compute_default_code(self):
         self.env.cr.flush()  # https://github.com/odoo/odoo/blob/16.0/odoo/models.py#L5592
         for rec in self:
             if not rec.manual_code:
-                rec.default_code = rec._generate_default_code()
+                new_code = rec._generate_default_code()
+                # Only write if the value actually changed to avoid unnecessary
+                # write_date updates that can cause PostgreSQL serialization errors
+                if rec.default_code != new_code:
+                    rec.default_code = new_code
 
     def _inverse_default_code(self):
         for rec in self:
@@ -230,6 +234,8 @@ def _generate_default_code(self):
         ):
             return None
         else:
+            if not self.product_tmpl_id.reference_mask:
+                return None
             product_attrs = defaultdict(str)
             reference_mask = ReferenceMask(self.product_tmpl_id.reference_mask)
             main_lang = self.product_tmpl_id._guess_main_lang()