Allow templates without alerts (#13150)

The confirmation page template system allows templates to have alerts,
but it throws an uncaught Promise rejection if a template has no
alerts. This is the unintended side-effect of input validation.

The `getTemplateAlerts` function was updated to always return an array.
The one callsite was updated to expect an empty array if there were no
alerts to render, rather than expecting `undefined`.

ui/pages/confirmation/confirmation.js

@@ -83,7 +83,7 @@ function useAlertState(pendingConfirmation) {
     let isMounted = true;
     if (pendingConfirmation) {
       getTemplateAlerts(pendingConfirmation).then((alerts) => {
-        if (isMounted && alerts) {
+        if (isMounted && alerts.length > 0) {
           dispatch({
             type: 'set',
             confirmationId: pendingConfirmation.id,

ui/pages/confirmation/templates/index.js

@@ -43,7 +43,7 @@ const ALLOWED_TEMPLATE_KEYS = [
  */
 export async function getTemplateAlerts(pendingApproval) {
   const fn = APPROVAL_TEMPLATES[pendingApproval.type]?.getAlerts;
-  const results = fn ? await fn(pendingApproval) : undefined;
+  const results = fn ? await fn(pendingApproval) : [];
   if (!Array.isArray(results)) {
     throw new Error(`Template alerts must be an array, received: ${results}`);
   }