nixos: Allow settings to be omitted with null

This allows for suppressing the default database.url setting
in the NixOS module.

In the future, we need to rethink about the precedence of
configuration sources. It makes sense for the environment
variables to take precedence over configuration files.

nixos/atticd.nix

@@ -15,6 +15,9 @@ let
   overlay = flake.defaultNix.overlays.default;
 
   format = pkgs.formats.toml { };
+  filteredSettings = lib.converge
+    (lib.filterAttrsRecursive (_: v: ! lib.elem v [{ } null]))
+    cfg.settings;
 
   checkedConfigFile =
     pkgs.runCommand "checked-attic-server.toml"
@@ -113,7 +116,17 @@ in
         description = ''
           Structured configurations of atticd.
         '';
-        type = format.type;
+        type = let
+          valueType = with types; nullOr (oneOf [
+            bool
+            int
+            float
+            str
+            path
+            (attrsOf valueType)
+            (listOf valueType)
+          ]);
+        in types.attrsOf valueType;
         default = { }; # setting defaults here does not compose well
       };
 
@@ -124,7 +137,7 @@ in
           By default, it's generated from `services.atticd.settings`.
         '';
         type = types.path;
-        default = format.generate "server.toml" cfg.settings;
+        default = format.generate "server.toml" filteredSettings;
         defaultText = "generated from `services.atticd.settings`";
       };