Coding Standards: Explicitly return null in wp_set_all_user_settings().

This matches the documented `@return` type.

Follow-up to [22256], [37263], [39932], [48320].

Props justlevine.
See #63268.

git-svn-id: https://develop.svn.wordpress.org/trunk@60361 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/option.php

@@ -1880,7 +1880,7 @@ function wp_set_all_user_settings( $user_settings ) {
 	}
 
 	if ( ! is_user_member_of_blog() ) {
-		return;
+		return null;
 	}
 
 	$settings = '';