[data] Fix WebAuthn Authenticate

[horus]

Hello,

I've noticed that

      <#if authenticators??>
        <form x-ref="authnSelectForm">
          <#list authenticators.authenticators as authenticator>
            <input value="${authenticator.credentialId}" type="hidden" />
          </#list>
        </form>

There are some circumstances which an user instance's not required (i.e., passwordless), thus authenticators will evaluate to null, which further cause the form being skipped entirely.

According to the code in webauthn-authenticate.ftl#L95, the authnSelectForm should be checked.
Otherwise it will result in error:

Uncaught TypeError: can't access property "elements", l is undefined

This PR tries to address the issue. Thank you!

[lukin]

Hey @horus 👋

Thank you for your contribution. I'll prepare a build for this fix.

[horus]

Glad I could be of help!
Thank you for creating and maintaining this.