Check for existence of data from `get_blogaddress_by_id()` before returning a URL

* Prevent a notice when an invalid ID is used with `get_blogaddres_by_id()`.
* Return a falsy empty string rather than the previous "http://".
* Add unit tests for `get_blogaddress_by_id()`.

Props nerrad.

Fixes #30566.

Built from https://develop.svn.wordpress.org/trunk@31157


git-svn-id: http://core.svn.wordpress.org/trunk@31138 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/ms-blogs.php

@@ -33,11 +33,11 @@ function wpmu_update_blogs_date() {
  * @since MU
  *
  * @param int $blog_id Blog ID
- * @return string
+ * @return string Full URL of the blog if found. Empty string if not.
  */
 function get_blogaddress_by_id( $blog_id ) {
 	$bloginfo = get_blog_details( (int) $blog_id, false ); // only get bare details!
-	return esc_url( 'http://' . $bloginfo->domain . $bloginfo->path );
+	return isset( $bloginfo->domain ) && isset( $bloginfo->path ) ?  esc_url( 'http://' . $bloginfo->domain . $bloginfo->path ) : '';
 }
 
 /**

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.2-alpha-31156';
+$wp_version = '4.2-alpha-31157';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.