phpbb\log should still work even when no user data is given

If system cron is active, there will not exist a user object which means that some cron tasks which aren't aware of this fact will lock cron because they throw a fatal error. See: https://www.phpbb.com/community/viewtopic.php?p=14302461#p14302461

Since this is an easy fix we should include it in the core to prevent locking cron.