add empty markup to TrackingCodeBlock

Closes #3336760

src/Plugin/Block/TrackingCodeBlock.php

@@ -67,7 +67,7 @@ class TrackingCodeBlock extends BlockBase implements ContainerFactoryPluginInter
     $tracking_code = $config->get('tracking_code');
     if (!empty($tracking_code)) {
       $tracking_code = preg_replace('#<script(.*?)>(.*?)</script>#is', '$2', $tracking_code);
-
+      $build['#markup'] = '';
       $build['#attached']['library'][] = 'apsisone/tracking';
       $build['#attached']['drupalSettings']['apsisone']['trackingcode'] = $tracking_code;
     }