Layout builder discards cacheability metadata from blocks when they are rendered without content

Problem/Motivation

Layout builder will discard cacheability metadata from blocks if they are visited and produce no content.

Digging into how core blocks function, it looks like this is explicitly captured:

\Drupal\block\BlockViewBuilder::preRender

      // If $content is not empty, then it contains cacheability metadata, and
      // we must merge it with the existing cacheability metadata. This allows
      // blocks to be empty, yet still bubble cacheability metadata, to indicate
      // why they are empty.

With the offending code in LB discarding the cacheability metadata in the same circumstance:

\Drupal\layout_builder\EventSubscriber\BlockComponentRenderArray::onBuildRender

      $content = $block->build();
      $is_content_empty = Element::isEmpty($content);
      $is_placeholder_ready = $event->inPreview() && $block instanceof PreviewFallbackInterface;
      // If the content is empty and no placeholder is available, return.
      if ($is_content_empty && !$is_placeholder_ready) {
        return;
      }

Proposed resolution

Make it consistent, capture the metadata from empty blocks and bubble it appropriately.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet