Mark WP_List_Table::bulk_actions()'s argument $which as optional …

…for BC.

Fixes #29026.

Built from https://develop.svn.wordpress.org/trunk@29440


git-svn-id: http://core.svn.wordpress.org/trunk@29218 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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

@@ -345,8 +345,9 @@ protected function get_bulk_actions() {
 	 * @access protected
 	 *
 	 * @param string $which The location of the bulk actions: 'top' or 'bottom'.
+	 *                      This is designated as optional for backwards-compatibility.
 	 */
-	protected function bulk_actions( $which ) {
+	protected function bulk_actions( $which = '' ) {
 		if ( is_null( $this->_actions ) ) {
 			$no_new_actions = $this->_actions = $this->get_bulk_actions();
 			/**