[FIX] purchase: Scheduled Date in PO

When setting the quantity in a PO line, the field "date_planned" is changed
by onchange_product_id and this field is equal to:
-the date of today + seller.delay if there is at least one provider for the product
-the date of today if there is no provider set for the product
-the date manually set if there is no provider and already a scheduled_date set

To keep the similar behaviour to 8.0 for product without provider.

opw:652394

addons/purchase/purchase.py

@@ -571,8 +571,8 @@ class PurchaseOrderLine(models.Model):
         if price_unit and seller and self.order_id.currency_id and seller.currency_id != self.order_id.currency_id:
             price_unit = seller.currency_id.compute(price_unit, self.order_id.currency_id)
         self.price_unit = price_unit
-
-        self.date_planned = self._get_date_planned(seller).strftime(DEFAULT_SERVER_DATETIME_FORMAT)
+        if seller or not self.date_planned:
+            self.date_planned = self._get_date_planned(seller).strftime(DEFAULT_SERVER_DATETIME_FORMAT)
 
         product_lang = self.product_id.with_context({
             'lang': self.partner_id.lang,