[FIX] web_editor: regression less editor 5af122387

In 5af12238 a change was made to have the correct hierarchy levels, but
this can cause an issue when editing less files.

This is because we now had "undefined + 1" which gives NaN which breaks
Array constructor.

opw-814579
opw-815397
opw-814322
opw-815265
opw-814021
closes #23006

addons/web_editor/static/src/js/widgets/ace.js

@@ -742,7 +742,7 @@ var ViewEditor = Widget.extend({
 
             var text = data.text || '';
             if (!isSelected) {
-                text = Array($elem.data('level') + 1).join('-') + ' ' + text;
+                text = Array(($elem.data('level') || 0) + 1).join('-') + ' ' + text;
             }
             var $div = $('<div/>',  {
                 text: text,