[FIX] product: Error message when adding an item to a quote

When a user tries to add a product on an opportunity, the date_order of the
order is added into the context. This value can be False.

When "_compute_price_rule" is called without "date" arguments
(some onchange, compute...) the method method use the context date (in this
case the value is False) or the today date.
The date value is used in an sql request and raise a value exception.

opw-751700

addons/product/models/product_pricelist.py

@@ -106,7 +106,7 @@ class Pricelist(models.Model):
         """
         self.ensure_one()
         if not date:
-            date = self._context.get('date', fields.Date.today())
+            date = self._context.get('date') or fields.Date.today()
         if not uom_id and self._context.get('uom'):
             uom_id = self._context['uom']
         if uom_id: