Fatal error on multiple rendering of already printed block with empty markup

Problem/Motivation

Within a templateregion--header.html.twig we render a block "manually" by calling " a custom block via {{ elements.my_project_block_plugin_1 }}. This leads to a fatal error in case the custom block plugin returns an empty markup string

  public function build() {
    // [...]
    return [
      '#markup' => '',
    ];
  }

:

Error: Call to a member function getPlugin() on null in Drupal\block\BlockViewBuilder::preRender() (line 203 of core/modules/block/src/BlockViewBuilder.php).

It appears that some circumstances lead to the block being rendered a second time, although it has been rendered before and has been marked as #printed already:

1. Region "header" is rendered in page.html.twig as {{ page.header }}.
2. This leads to all children being render processed and concatenated to #children beforehand.
3. As the region template itself renders the child directly via {{ elements.my_project_block_plugin_1 }}, the block is passed to \Drupal\Core\Template\TwigExtension::escapeFilter() once again.
4. There the check on the element being already renders fails, as it also checks on strlen($arg['#markup']) > 0.
5. This leads to the re-rendering of the block - at least the try of it.

From my perspective this seems like a generic bug, as this may lead to multiple render processing of elements that simply will fall back to an empty string.

Proposed resolution

* Remove the general check on strlen($arg['#markup']) in

// Early return if this element was pre-rendered (no need to re-render).
if (isset($arg['#printed']) && $arg['#printed'] == TRUE && isset($arg['#markup']) && strlen($arg['#markup']) > 0) {
   return $arg['#markup'];
}

appearing in \Drupal\Core\Template\TwigExtension::escapeFilter() and \Drupal\Core\Template\TwigExtension::renderVar()

Remaining tasks

* Write a test.
* Clarify what the intensions were in #2464045: Move twig_render_var/twig_drupal_escape_filter to TwigExtension, inject the renderer in Twig extension and inline render() / show() function instead of calling it of adding strlen($arg['#markup']).
* Fix

User interface changes

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API changes

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Data model changes

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