Merge pull request cachix#426 from cachix/fix-default-package

Set the default hook package to null

Author: domenkozar

modules/hook.nix

@@ -46,6 +46,7 @@ in
 
     package = mkOption {
       type = types.nullOr types.package;
+      default = null;
       description = lib.mdDoc
         ''
           An optional package that provides the hook.

modules/pre-commit.nix

@@ -138,7 +138,7 @@ in
             Useful for including into the developer environment.
           '';
 
-        default = builtins.map (hook: hook.package) (lib.filter (hook: hook.enable) (builtins.attrValues config.hooks));
+        default = builtins.map (hook: hook.package) (lib.filter (hook: hook.enable && hook.package != null) (builtins.attrValues config.hooks));
       };
 
       hooks =