Fix serializable_classes handling when config is false

When serializable_classes is false (Laravel 13 default), the guard
treated it the same as null/true and skipped registration. But false
means "restrict to no classes", so unserialize() blocked all Stache
objects. Now only null and true (both unrestricted) are skipped.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jasonvarga

src/Providers/AddonServiceProvider.php

@@ -721,11 +721,11 @@ protected function registerSerializableClasses(array $classes)
     {
         $existing = $this->app['config']->get('cache.serializable_classes');
 
-        if (! is_array($existing)) {
+        if ($existing === null || $existing === true) {
             return;
         }
 
-        $this->app['config']->set('cache.serializable_classes', array_merge($existing, $classes));
+        $this->app['config']->set('cache.serializable_classes', array_merge(is_array($existing) ? $existing : [], $classes));
     }
 
     protected function schedule(Schedule $schedule)

src/Providers/AppServiceProvider.php

@@ -239,11 +239,11 @@ private function registerSerializableClasses()
     {
         $existing = $this->app['config']->get('cache.serializable_classes');
 
-        if (! is_array($existing)) {
+        if ($existing === null || $existing === true) {
             return;
         }
 
-        $this->app['config']->set('cache.serializable_classes', array_merge($existing, [
+        $this->app['config']->set('cache.serializable_classes', array_merge(is_array($existing) ? $existing : [], [
             \Statamic\Auth\File\User::class,
             \Statamic\Assets\Asset::class,
             \Statamic\Assets\AssetContainer::class,