Multisite: Set `WP_Network` `blog_id` property default to string as expected.

The `blog_id` property is always returned (and expected) as a string, so we should set it as one by default.

Props flixos90.
See #36717.


git-svn-id: https://develop.svn.wordpress.org/trunk@37871 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -63,7 +63,7 @@ class WP_Network {
 	 * @access private
 	 * @var string
 	 */
-	private $blog_id = 0;
+	private $blog_id = '0';
 
 	/**
 	 * Domain used to set cookies for this network.