Don't render empty blocks

Fix

Add an event that can be used by developers to add logic that will control visibility of a block e.g check for view blocks who have empty content.

For example

class HomeboxSubscriber implements EventSubscriberInterface {

  public static function getSubscribedEvents() {
    $events[HomeboxEvents::BLOCK] = 'onBlockPlacement';
    return $events;
  }

  /**
   * Checks if block should be placed on the page.
   *
   * @param \Drupal\homebox\Event\HomeboxBlockEvent $event
   *   The homebox block event.
   */
  public function onBlockPlacement(HomeboxBlockEvent $event) {

    if($event->getBlock() instanceof ViewsBlock) {
      $noContent = !isset($event->getRenderArray()['render']['#block_content']['view_build']) || empty($event->getRenderArray()['render']['#block_content']['view_build']['#rows']);
      if($noContent && !in_array('administrator', $event->getAccount()->getRoles())) {
        $event->setIsEmpty(TRUE);
      }
    }
  }

}