system.matchuserdata with a blank name matches an existing user. For example: system.matchuserdata(U_NAME, "")
I'm currently running Synchronet on Linux with binaries built this morning (January 22, 2023). I've noticed that when I call system.matchuserdata(U_NAME, ""), it matches an existing user. For me it's choosing user number 3, and that user account has a non-blank real name (and alias), so I'm not sure why it's choosing that particular user.