Docs: Correct the type for WP_List_Table::$_column_headers.

The value is `null` until `::get_column_info()` is called.

Follow-up to [31127], [35021].

Props justlevine.
See #63268.

git-svn-id: https://develop.svn.wordpress.org/trunk@60292 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/class-wp-list-table.php

@@ -72,10 +72,10 @@ class WP_List_Table {
 	protected $modes = array();
 
 	/**
-	 * Stores the value returned by ->get_column_info().
+	 * Stores the value returned by ::get_column_info().
 	 *
 	 * @since 4.1.0
-	 * @var array
+	 * @var array|null
 	 */
 	protected $_column_headers;