Coerce optionset label to string before Html::escape() in
getOptionsetsByGroupOptions(). PHP 8.1+ rejects null for string,
and a Splide optionset config entity with a missing/null label
(e.g. from a malformed config import or a partial slick→splide
migration) crashes any admin form that lists optionsets, including
/admin/structure/block.

--- a/src/Form/SplideAdmin.php
+++ b/src/Form/SplideAdmin.php
@@ -471,7 +471,7 @@ class SplideAdmin implements SplideAdminInterface {
     $optionsets = $groups = $ungroups = [];
     $splides = $this->manager->loadMultiple('splide');
     foreach ($splides as $splide) {
-      $name = Html::escape($splide->label());
+      $name = Html::escape((string) $splide->label());
       $id = $splide->id();
       $current_group = $splide->getGroup();
       if (!empty($group)) {