drupal_block() show fatal error if block plugin return NULL

Problem/Motivation

Some blocks plugins return NULL in build() method, eg \Drupal\aggregator\Plugin\Block\AggregatorFeedBlock::build(). Because of this, an fatal error:

TypeError: Argument 1 passed to Drupal\Core\Render\Element::isEmpty() must be of the type array, null given, called in modules\contrib\twig_tweak\src\TwigExtension.php on line 476 in Drupal\Core\Render\Element::isEmpty() (line 198 of core\lib\Drupal\Core\Render\Element.php).

Text error from 2.x version

Steps to reproduce

1. Create block plugin with:

public function build() {
  return NULL;
}

2. Add in twig file:

{{ drupal_block('block_with_null') }}