@Override
    public Optional<Credential> getInitialCredential() {
        if (this.providedCredentials.isEmpty()) {
            LOGGER.warn("Provided credentials chain is empty as no credentials have been collected");
        }
        return providedCredentials.stream().findFirst();
    }
Principal id is and must be enforced to be the same for all authentications.
Based on that restriction, it's safe to grab the first principal id in the chain
when composing the authentication chain for the caller.
core/cas-server-core-authentication-api/src/main/java/org/apereo/cas/authentication/DefaultAuthenticationResultBuilder.java