Global styles experiment: return early (#45929)

Author: rcrdortiz

projects/packages/jetpack-mu-wpcom/changelog/dotcom-14917-global-styles-on-personal-bridge-explat-and-the-gating-logic-owner

@@ -0,0 +1,4 @@
+Significance: patch
+Type: security
+
+Implemented a guard against non user objects when assigning an experiment variation

projects/packages/jetpack-mu-wpcom/src/features/wpcom-global-styles/index.php

@@ -818,6 +818,9 @@ function is_global_styles_on_personal_plan() {
 			return false;
 		}
 		$wpcom_blog_owner = get_userdata( $wpcom_blog_owner_id );
+		if ( ! $wpcom_blog_owner ) {
+			return false;
+		}
 		// WP.com: Direct ExPlat assignment.
 		$assignment = \ExPlat\assign_given_user( $experiment_key, $wpcom_blog_owner );
 		$enabled    = ( null !== $assignment );