[FIX] stock_barcode: correctly check when to execute "O-BTN"

It's possible to trigger click on form view's button with the help
of the "barcode_trigger" attributes. However, we don't want to be
able to execute clicks on items that aren't clickable by the user.
That's why we wrote the following condition: if the elem is visible
OR is a child of a .dropdown-menu, we trigger the click.

The second condition was always true since we used the jquery's
parent function which return an empty list when no result, and
an empty list is truthy.

addons/barcodes/static/src/js/inherited_form_widget_button.js

@@ -16,7 +16,7 @@ var ButtonBarcodeHandlerMixin = _.extend({}, BarcodeHandlerMixin, {
             this.on_barcode_scanned = function(barcode) {
                 var match = barcode.match(/O-BTN\.(.+)/);
                 if (match && match[1] === self.node.attrs.barcode_trigger &&
-                    (self.$el.is(':visible') || self.$el.parent('.dropdown-menu'))) {
+                    (self.$el.is(':visible') || self.$el.parent('.dropdown-menu').length)) {
                     self.on_click();
                 }
             };