Coding Standards: Correct $site_id parameter default values in WP_User.

This commit corrects the default `$site_id` parameter values used throughout the `WP_User` class to bring them in line with their `int` doctypes (and the doctypes of the functions used internally by the affected class methods).

More specifically,
* `__construct()`: `$site_id = ''` changed to `$site_id = 0`
* `init()`: `$site_id = ''` changed to `$site_id = 0`
* `for_blog()`: `$blog_id = ''` changed to `$blog_id = 0`
* `for_site()`: `$site_id = ''` changed to `$site_id = 0`

Follow-up to [12796], [15566], [18597], [41624].

Props justlevine.
See #63268.

git-svn-id: https://develop.svn.wordpress.org/trunk@60174 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-user.php

@@ -125,7 +125,7 @@ class WP_User {
 	 * @param string                      $name    Optional. User's username
 	 * @param int                         $site_id Optional Site ID, defaults to current site.
 	 */
-	public function __construct( $id = 0, $name = '', $site_id = '' ) {
+	public function __construct( $id = 0, $name = '', $site_id = 0 ) {
 		global $wpdb;
 
 		if ( ! isset( self::$back_compat_keys ) ) {
@@ -175,7 +175,7 @@ class WP_User {
 	 * @param object $data    User DB row object.
 	 * @param int    $site_id Optional. The site ID to initialize for.
 	 */
-	public function init( $data, $site_id = '' ) {
+	public function init( $data, $site_id = 0 ) {
 		if ( ! isset( $data->ID ) ) {
 			$data->ID = 0;
 		}
@@ -852,7 +852,7 @@ class WP_User {
 	 *
 	 * @param int $blog_id Optional. Site ID, defaults to current site.
 	 */
-	public function for_blog( $blog_id = '' ) {
+	public function for_blog( $blog_id = 0 ) {
 		_deprecated_function( __METHOD__, '4.9.0', 'WP_User::for_site()' );
 
 		$this->for_site( $blog_id );
@@ -867,7 +867,7 @@ class WP_User {
 	 *
 	 * @param int $site_id Site ID to initialize user capabilities for. Default is the current site.
 	 */
-	public function for_site( $site_id = '' ) {
+	public function for_site( $site_id = 0 ) {
 		global $wpdb;
 
 		if ( ! empty( $site_id ) ) {