Site Health: Set missing $transient in WP_Debug_Data:get_wp_themes_inactive().

This commit addresses a bug where the `update_themes` site transient was not defined before being checked in several `isset()` checks later on in the method.

Follow-up to [59176].

Props justlevine.
See #63268.

git-svn-id: https://develop.svn.wordpress.org/trunk@60181 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/class-wp-debug-data.php

@@ -1300,6 +1300,7 @@ private static function get_wp_themes_inactive(): array {
 		$active_theme  = wp_get_theme();
 		$parent_theme  = $active_theme->parent();
 		$theme_updates = get_theme_updates();
+		$transient     = get_site_transient( 'update_themes' );
 
 		$auto_updates         = array();
 		$auto_updates_enabled = wp_is_auto_update_enabled_for_type( 'theme' );