If `$blog_id` equals `0`, it should be treated as the current site ID, and there is no need to switch to a different site.
This commit prevents an unnecessary call to `switch_to_blog()` on single site to avoid a fatal error when using `'orderby' => 'post_count'` and the deprecated `'who' => 'authors'` parameter:
{{{
Uncaught Error: Call to undefined function switch_to_blog() in wp-includes/class-wp-user-query.php:1077
}}}
Follow-up to [55657].
Props dd32, austinginder, RavanH, mukesh27.
Fixes#59011.
Built from https://develop.svn.wordpress.org/trunk@56381
git-svn-id: http://core.svn.wordpress.org/trunk@55893 1a063a9b-81f0-0310-95a4-ce76da25c4cd
f106155bf8