Docs: Correct @type annotation for WP_Customize_Selective_Refresh::$current_partial_id.

Props diddledan.
Fixes #46854.

git-svn-id: https://develop.svn.wordpress.org/trunk@45428 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/customize/class-wp-customize-selective-refresh.php

@@ -49,7 +49,7 @@ final class WP_Customize_Selective_Refresh {
 	 * Keep track of the current partial being rendered.
 	 *
 	 * @since 4.5.0
-	 * @var string
+	 * @var string|null
 	 */
 	protected $current_partial_id;