Stringify the default_value

$process_line[0]['default_value'] isn't always a string eg migration 'd7_field_formatter_settings' has a default of [].

src/Controller/MigrationController.php

@@ -186,7 +186,11 @@ class MigrationController extends ControllerBase implements ContainerInjectionIn
         $row[] = '';
       }
       if (isset($process_line[0]['default_value'])) {
-        $row[] = ['data' => Xss::filterAdmin($process_line[0]['default_value'])];
+        $default = $process_line[0]['default_value'];
+        if (!is_string($default)) {
+          $default = print_r($default, TRUE);
+        }
+        $row[] = ['data' => Xss::filterAdmin($default)];
       }
       else {
         $row[] = '';