vdirsyncer: set postHook to null when not set

The `postHook` option was being processed and reset to a string, even if
the user set it to null, causing issues under certain conditions (see

Using `if-then-else` instead of `optionalString` keeps the option as
null, instead of setting it to an empty string.

modules/programs/vdirsyncer.nix

@@ -26,9 +26,11 @@ let
     filterAttrs (_: v: v != null)
     ((getAttrs [ "type" "fileExt" "encoding" ] a.local) // {
       path = a.local.path;
-      postHook = optionalString (a.vdirsyncer.postHook != null)
+      postHook = if a.vdirsyncer.postHook != null then
         (pkgs.writeShellScriptBin "post-hook" a.vdirsyncer.postHook
-          + "/bin/post-hook");
+          + "/bin/post-hook")
+      else
+        null;
     });
 
   remoteStorage = a: