[FIX] pos_restaurant: sync empty orders

* In a pos-session with table management create a new order with products
and go back to the floor-plan.
* The order is sinced to the back-end.
* Open the table again, remove all products but keep the order.
* Go back to floorplan to trigger syncing again.
* Open the table again.

behaviour before fix:
* The empty order is not loaded in the frontend anymore.
* To close the session the order has to be manually removed in the backend.

After fix:
* The order is still visible in the frontend and can be removed or finalysed as expected.

closes odoo/odoo#40165

X-original-commit: 3add8dd226f39d5166873a575b858694eeb2e6ee
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/pos_restaurant/static/src/js/floors.js

@@ -959,11 +959,9 @@ models.PosModel = models.PosModel.extend({
                         }
                     });
                     server_orders.forEach(function(server_order){
-                        if (server_order.lines.length){
-                            var new_order = new models.Order({},{pos: self, json: server_order});
-                            self.get("orders").add(new_order);
-                            new_order.save_to_db();
-                        }
+                        var new_order = new models.Order({},{pos: self, json: server_order});
+                        self.get("orders").add(new_order);
+                        new_order.save_to_db();
                     })
                     if (!ids_to_remove.length) {
                         self.set_synch('connected');

addons/pos_restaurant/static/src/js/tours/pos_restaurant.js

@@ -201,6 +201,32 @@ odoo.define('pos_reataurant.tour.synchronized_table_management', function (requi
     // Test transfering an empty order
     steps = steps.concat(transfer_order_to_table('T4', '2'));
 
+    steps = steps.concat(add_product_to_order('Coca-Cola'));
+    steps = steps.concat(verify_order_total('2.20'));
+
+    // Take a synced order with products, remove the products 
+    // and check if the order is still available in the front-end
+    steps = steps.concat([{
+        content: 'back to floor',
+        trigger: '.floor-button',
+        run: 'click',
+    }]);
+    steps = steps.concat(open_table('T4', 1));
+    steps = steps.concat([{
+        content: 'click backspace to set quantity to 0',
+        trigger: '.numpad-backspace',
+        run: 'click',
+    }, {
+        content: 'click backspace to remove line',
+        trigger: '.numpad-backspace',
+        run: 'click',
+    }]);
+    steps = steps.concat([{
+        content: 'back to floor',
+        trigger: '.floor-button',
+        run: 'click',
+    }]);
+    steps = steps.concat(open_table('T4', 1));
     steps = steps.concat(add_product_to_order('Coca-Cola'));
     steps = steps.concat(verify_order_total('2.20'));
     steps = steps.concat([{