Use null instead of an empty string for closed sidebar

[MAILPOET-2450]

assets/js/src/form_editor/components/editor.jsx

@@ -26,7 +26,7 @@ export default () => {
         </div>
       </div>
       <div>
-        { sidebarOpened ? <Sidebar /> : '' }
+        { sidebarOpened ? <Sidebar /> : null }
       </div>
     </div>
   );