navbar: fix null check in the javascript

js/navbar.js

@@ -18,7 +18,7 @@ function ubuntuit_navbar_center_active() {
     var navs = document.querySelectorAll("nav");
     for (var i = 0; i < navs.length; i++) {
         var active = navs[i].querySelector(".active");
-        if (active !== undefined) {
+        if (active !== null) {
             var offset = active.getBoundingClientRect().left;
             navs[i].scrollBy(offset - window.innerWidth / 2 + active.offsetWidth / 2, 0);
         }