Nullify / unset `default-user` and `default-password` in client configuration?

[tomeon]

❓ Nullify / unset default-user and default-password in client configuration ❓

First, thanks for ntfy -- it was easy to set up a self-hosted instance and has been a real treat to use.

I am using ntfy version 2.5.0 on NixOS 23.05. Here's my situation, slightly fictionalized for brevity and security:

--- 
# ~/.config/ntfy/client.yml

default-user: myuser
default-password: hunter2
default-host: https://ntfy.local # self-hosted, LAN-only
default-command: issue-desktop-notification

subscribe:
  - topic: thing
  - topic: other-thing
  - topic: yet-another-thing
  - topic: https://ntfy.sh/needs-to-use-public-host-for-reasons
    user: ??? # don't use `user` for public instance
    password: ??? # don't use `password` for public instance

My question is: what, if anything, can I use in place of the ???s above in order to disable authentication for the topic https://ntfy.sh/needs-to-use-public-host-for-reasons? I've tried using each of the four combinations in the Cartesian product of {"", null} x {"", null} for user and password, but none them work (which is to say that I continue to get 401s from ntfy sub --from-config).

Am I holding it wrong? Or is this not possible at present? I can of course list user and password for each of the subscriptions on my self-hosted instance, which is effective but cumbersome.

Thank you!