List tables: Even more robust fallback for the primary column.

Since `WP_List_Table::get_default_primary_column()` can be overridden in a subclass and columns can be further filtered, it is possible to end up with a primary column that no longer exists. This is particularly evident in list tables for custom post types that have been altered via filter.

props ocean90.
fixes #32996.


git-svn-id: https://develop.svn.wordpress.org/trunk@33266 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -1069,7 +1069,7 @@ class WP_Posts_List_Table extends WP_List_Table {
 	 * @return string Name of the default primary column, in this case, 'title'.
 	 */
 	protected function get_default_primary_column_name() {
-		return( 'title' );
+		return 'title';
 	}
 
 	/**