nixos/openssh: allow removing settings

# Motivation
So far it was not possible to configure sshd to allow password authentication only for a specific user. This is because in the generated config a `Match User xxx` section would be required before the global `PasswordAuthentication` is defined, as otherwise the global option always takes precedence.
The same problem occurs with multiple other options under `settings`.

# Done
This PR fixes that issue for all settings by simply allowing them to be overridden with `null`, which leads to a removal of that setting from the config.
The user can then correctly configure user specific settings using extraConfig, like this:
```
    Match User user1
    PasswordAuthentication yes
    Match all
    PasswordAuthentication no
```

0 files changed, 0 insertions, 0 deletions