Coding Standards: Explicitly return null in wp_get_nav_menu_to_edit().

This commit updates the function to explicitly return `null` if a valid `$menu_id` is not passed or the term does not exist. Previously, the function could return `void`, but was typed as only returning a string or a `WP_Error` object.

Follow-up to [14248].

Props justlevine.
See #63268.

git-svn-id: https://develop.svn.wordpress.org/trunk@60180 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/nav-menu.php

@@ -1239,7 +1239,8 @@ function _wp_nav_menu_meta_box_object( $data_object = null ) {
  * @since 3.0.0
  *
  * @param int $menu_id Optional. The ID of the menu to format. Default 0.
- * @return string|WP_Error The menu formatted to edit or error object on failure.
+ * @return string|WP_Error|null The menu formatted to edit or error object on failure.
+ *                              Null if the `$menu_id` parameter is not supplied or the term does not exist.
  */
 function wp_get_nav_menu_to_edit( $menu_id = 0 ) {
 	$menu = wp_get_nav_menu_object( $menu_id );
@@ -1321,6 +1322,8 @@ function wp_get_nav_menu_to_edit( $menu_id = 0 ) {
 	} elseif ( is_wp_error( $menu ) ) {
 		return $menu;
 	}
+
+	return null;
 }
 
 /**