Set the default hook package to null

Allows creating hooks that don't depend on a package without having to
explicitly set this to null.

Author: sandydoo

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.