Drupal 8 migration validate `NULL` since is read as string

migration-event-class.php

<?php

// ...

public function onPrepareRow(MigratePrepareRowEvent $event) {
  $row = $event->getRow();
  $fields = $event->getMigration()->getPluginDefinition()['process'];
  foreach ($fields as $destination => $source) {
    if (!is_array($destination) && !is_array($source)) {
      if ($row->getSourceProperty($source) == "NULL") {
        $row->setSourceProperty($source, NULL);
      }
    }
  }
}

// ...

@heddn awesome I will make the changes and reply you back.