Yeah, you can pass null or an empty string.
Additionally:
1. Don't mark channels as persistent (so they will be destroyed when the last player leaves)
2. Don't use TNManager.SetPlayerSave
This should result in only a few files being saved: ban.txt, admin.txt, and world.config.
'world' is just the default name of the config file, in this case it would function more as a general server config as it doesn't store any world-specific data (unless you use TNManager.serverData or SetServerData).
It's worth noting that RFCs will still "save" on the channels during *runtime*, they just won't be saved to the filesystem on exit. So Target.AllSaved, Target.OthersSaved, etc will still work correctly if you need them.