[FIX] product: prevent ValueError when computing quantity on a void recordset

Invoice lines do not necessarily have a product and therefore an UoM set
on them. You could then compute quantityes on a void recordset. In that
case we just return the quantity untouched.

This is actually the old behavior of compute_quantity that is set back.
See https://github.com/odoo/odoo/blob/01279359b9e5340d5ecb15f0fee186f039356374/addons/product/product.py#L164 .

Closes #16377

addons/product/models/product_uom.py

@@ -90,6 +90,8 @@ class ProductUoM(models.Model):
 
     @api.multi
     def _compute_quantity(self, qty, to_unit, round=True, rounding_method='UP'):
+        if not self:
+            return qty
         self.ensure_one()
         if self.category_id.id != to_unit.category_id.id:
             if self._context.get('raise-exception', True):