Issue #3191574: Prevent a PHP warning on cache. clear

includes/theme.inc

@@ -17,7 +17,7 @@ function bootstrap_storybook_library_info_build() {
   $extensions = array_map('preg_quote', $extensions);
   $extensions = implode('|', $extensions);
   if (!is_dir($directory)) {
-    return;
+    return [];
   }
   $file_scan = \Drupal::service('file_system')->scanDirectory($directory, "/{$extensions}$/");
   $libraries = [];