refs #18034 [finc] check for null in cart-finc

themes/finc/js/cart-finc.js

 /* #18034 - inform screen reader about changes, eventually intregrate by PR into bootstrap cart.js - RL */
 $(document).ready(function() {
   var cart = document.getElementById("cartSummary");
-  if (cart !== undefined) {
+  if (cart != null) {
     cart.setAttribute("aria-live", "polite");
     cart.setAttribute("aria-atomic", "true");
   }