Docs: Update the return notation for `get_current_screen()` to note that `null` can also be returned if the screen has not been defined.

Props screamingdev.
Fixes #36382.


git-svn-id: https://develop.svn.wordpress.org/trunk@37534 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/screen.php

@@ -200,7 +200,7 @@ function add_screen_option( $option, $args = array() ) {
  *
  * @global WP_Screen $current_screen
  *
- * @return WP_Screen Current screen object
+ * @return WP_Screen|null Current screen object or null when screen not defined.
  */
 function get_current_screen() {
 	global $current_screen;