Fix EZP-23176: Sessions are always started for anonymous user

JIRA: https://jira.ez.no/browse/EZP-23176

Problem:
Legacy's `eZUser::setCurrentlyLoggedInUser()` always starts a session, to store the `eZUserLoggedInID`.

This PR skips calling this function if the current user is anonymous, as legacy's session will still be able to pick this user by default (@see instance())

Note:
Additional or alternate fix (in legacy) would be to make sure `eZUser::setCurrentlyLoggedInUser()` does not start a new session.