jq: make colors and package nullable

...and since according to commit
f9b5172d95 the defaults are the upstream
values, default `colors` to null.

Setting JQ_COLORS is just unnecessary environment clutter when using the
default colors.

Although the module now does nothing besides (optionally) installing
`jq` when `colors` is left at its default value, that is still useful:
from a flake with multiple home-manager configurations, some of which
have `jq` provided through other means, setting/forcing
`programs.jq.package` to null is more convenient than conditionally
adding `jq` to `home.packages`.

modules/programs/jq.nix

@@ -35,11 +35,11 @@ in
     programs.jq = {
       enable = lib.mkEnableOption "the jq command-line JSON processor";
 
-      package = lib.mkPackageOption pkgs "jq" { };
+      package = lib.mkPackageOption pkgs "jq" { nullable = true; };
 
       colors = mkOption {
         description = ''
-          The colors used in colored JSON output.
+          The colors used in colored JSON output, or null to use the defaults.
 
           See the [Colors section](https://jqlang.github.io/jq/manual/#Colors)
           of the jq manual.
@@ -58,30 +58,21 @@ in
           }
         '';
 
-        default = {
-          null = "1;30";
-          false = "0;37";
-          true = "0;37";
-          numbers = "0;37";
-          strings = "0;32";
-          arrays = "1;37";
-          objects = "1;37";
-          objectKeys = "1;34";
-        };
+        default = null;
 
-        type = colorsType;
+        type = types.nullOr colorsType;
       };
     };
   };
 
   config = mkIf cfg.enable {
-    home.packages = [ cfg.package ];
+    home.packages = lib.mkIf (cfg.package != null) [ cfg.package ];
 
     home.sessionVariables =
       let
         c = cfg.colors;
       in
-      {
+      lib.mkIf (c != null) {
         JQ_COLORS = "${c.null}:${c.false}:${c.true}:${c.numbers}:${c.strings}:${c.arrays}:${c.objects}:${c.objectKeys}";
       };
   };