nixos/znc: Fix config generator for certain null values

The type of ZNC's config option specifies that a configuration like

  config.User.paul = null;

should be valid, which is useful for clearing/disabling property sets
like Users and Networks. However until now the config generator
implementation didn't actually cover null values, meaning you'd get an
error like

  error: value is null while a set was expected, at /foo.nix:29:10

This fixes the implementation to correcly allow clearing of property
sets.