[IMP] purchase: create purchase order without any purchase order lines.

On purchase orders, the scheduled date is mantadory. This date is computed from
the purchase order lines and hidden. If there is no purchase order line, the date
stays empty and it's not possible to save the PO.
This commit uses the order date as scheduled date if there are no PO lines

addons/purchase/models/purchase.py

@@ -32,7 +32,7 @@ class PurchaseOrder(models.Model):
                 'amount_total': amount_untaxed + amount_tax,
             })
 
-    @api.depends('order_line.date_planned')
+    @api.depends('order_line.date_planned', 'date_order')
     def _compute_date_planned(self):
         for order in self:
             min_date = False
@@ -41,6 +41,8 @@ class PurchaseOrder(models.Model):
                     min_date = line.date_planned
             if min_date:
                 order.date_planned = min_date
+            else:
+                order.date_planned = order.date_order
 
     @api.depends('state', 'order_line.qty_invoiced', 'order_line.qty_received', 'order_line.product_qty')
     def _get_invoiced(self):