[FIX] stock: fix null value for product tracking

Due to the recent changes of precompute.
The `tracking` fields of `product.template` would never be filled with
the default 'none' when the module was not loaded.
This means that any new product.product demo/data would fail the upgrade
scripts.

TaskId-2675382

addons/stock/models/product.py

@@ -638,7 +638,7 @@ class ProductTemplate(models.Model):
         ('serial', 'By Unique Serial Number'),
         ('lot', 'By Lots'),
         ('none', 'No Tracking')],
-        string="Tracking", required=True,
+        string="Tracking", required=True, default='none', # Not having a default value here causes issues when migrating.
         compute='_compute_tracking', store=True, readonly=False, precompute=True,
         help="Ensure the traceability of a storable product in your warehouse.")
     description_picking = fields.Text('Description on Picking', translate=True)