Docs: Correct the type for WP_Screen::$_screen_settings.

This reflects the property's initial `null` state prior to initialization.

Follow-up to [55693], [61300].

Props Chouby, arkaprabhachowdhury, SergeyBiryukov.
Fixes #56607.

git-svn-id: https://develop.svn.wordpress.org/trunk@62837 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/class-wp-screen.php

@@ -89,7 +89,7 @@ final class WP_Screen {
 	 * have a `$parent_base` of 'edit'.
 	 *
 	 * @since 3.3.0
-	 * @var string|null
+	 * @var ?string
 	 */
 	public $parent_base;
 
@@ -99,7 +99,7 @@ final class WP_Screen {
 	 * Some `$parent_file` values are 'edit.php?post_type=page', 'edit.php', and 'options-general.php'.
 	 *
 	 * @since 3.3.0
-	 * @var string|null
+	 * @var ?string
 	 */
 	public $parent_file;
 
@@ -186,7 +186,7 @@ final class WP_Screen {
 	 * Stores the 'screen_settings' section of screen options.
 	 *
 	 * @since 3.3.0
-	 * @var string
+	 * @var ?string
 	 */
 	private $_screen_settings;