[FIX] point_of_sale: wait on invoicing to finish before printing receipt

After this commit was introduced 4a72a70302 , When you click on validate with invoicing enabled without customer,
the popup for select a customer open and immediately we switch to the  receipt screen, that gets rid of popup.
In this commit we print the receipt after invoice process done in backend when validate an order with invoicing enabled so that it's work like before.

addons/point_of_sale/static/src/js/screens.js

@@ -1911,12 +1911,13 @@ var PaymentScreenWidget = ScreenWidget.extend({
 
             invoiced.done(function(){
                 self.invoicing = false;
+                self.gui.show_screen('receipt');
             });
         } else {
             this.pos.push_order(order);
+            this.gui.show_screen('receipt');
         }
 
-        this.gui.show_screen('receipt');
     },
 });
 gui.define_screen({name:'payment', widget: PaymentScreenWidget});