Fix axum example for credential creation (#361)

While investigating #351 I noticed that the `excludeCredentials` id's were still in base64url and which was failing validation in 1Password. Here I mirror the same modifications done to the `allowCredentials` list.

This also adds an optional chaining to the `allowCredentials` list in
the request options.

tutorial/server/axum/assets/auth.js

@@ -12,6 +12,9 @@ function register () {
  .then(credentialCreationOptions => {
  credentialCreationOptions.publicKey.challenge = Base64.toUint8Array(credentialCreationOptions.publicKey.challenge);
  credentialCreationOptions.publicKey.user.id = Base64.toUint8Array(credentialCreationOptions.publicKey.user.id);
+ credentialCreationOptions.publicKey.excludeCredentials?.forEach(function (listItem) {
+ listItem.id = Base64.toUint8Array(listItem.id)
+ });
 
  return navigator.credentials.create({
  publicKey: credentialCreationOptions.publicKey
@@ -57,7 +60,7 @@ function login() {
  .then(response => response.json())
  .then((credentialRequestOptions) => {
  credentialRequestOptions.publicKey.challenge = Base64.toUint8Array(credentialRequestOptions.publicKey.challenge);
- credentialRequestOptions.publicKey.allowCredentials.forEach(function (listItem) {
+ credentialRequestOptions.publicKey.allowCredentials?.forEach(function (listItem) {
  listItem.id = Base64.toUint8Array(listItem.id)
  });