Fix custom DC for feature check (#4930)

This change plays it safer on the feature check such that if there are
no feature flags (like in custom dc instances), this will just return
false.

static/js/shared/feature_flags/util.ts

@@ -26,7 +26,7 @@ export const SCROLL_TO_TOP_FEATURE_FLAG = "scroll_to_top_button";
  * @returns Bool describing if the feature is enabled
  */
 export function isFeatureEnabled(featureName: string): boolean {
-  if (featureName in FEATURE_FLAGS) {
+  if (FEATURE_FLAGS && featureName in FEATURE_FLAGS) {
     return FEATURE_FLAGS[featureName];
   }
   return false;