Issue #3344504 by tr: Fix test failures

Closes #3344504

src/Plugin/views/style/Table.php

 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\views_aggregator\Plugin\views\style;
 
 use Drupal\Component\Utility\Xss;
@@ -547,7 +549,7 @@ class Table extends ViewsTable {
         // $aggr_values is indexed by group value and/or 'column'.
         // 'column' is the last evaluated value for the field.
         if (isset($aggr_values['column'])) {
-          $field_handlers[$field_name]->last_render = $aggr_values['column'];
+          $field_handlers[$field_name]->last_render = (string) $aggr_values['column'];
         }
         foreach ($aggr_values as $group => $value) {
           // 'column' function is last so may override earlier value.