REST API: Properly display JSON encoding errors.

Previously `null` was returned because the server was accessing the incorrect variable.

Props atimmer, technosailor.
Fixes #52106.

Built from https://develop.svn.wordpress.org/trunk@49918


git-svn-id: http://core.svn.wordpress.org/trunk@49617 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api/class-wp-rest-server.php

@@ -530,7 +530,7 @@ class WP_REST_Server {
 				);
 
 				$result = $this->error_to_response( $json_error_obj );
-				$result = wp_json_encode( $result->data[0] );
+				$result = wp_json_encode( $result->data );
 			}
 
 			if ( $jsonp_callback ) {