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.

nixos/modules/services/networking/znc/default.nix

@@ -47,11 +47,11 @@ let
           #     Baz=baz
           #     Qux=qux
           #   </Foo>
-          set = concatMap (subname: [
+          set = concatMap (subname: optionals (value.${subname} != null) ([
               "<${name} ${subname}>"
             ] ++ map (line: "\t${line}") (toLines value.${subname}) ++ [
               "</${name}>"
-            ]) (filter (v: v != null) (attrNames value));
+            ])) (filter (v: v != null) (attrNames value));
 
         }.${builtins.typeOf value};