Layout builder does not correctly bubble up cache metadata for empty blocks

Problem/Motivation

When layout builder constructs the render array for a block component, it calls the build() method on the block plugin instance and checks if it's empty (using Element::isEmpty()). If it is empty, then it returns early without returning any render data to the block component. This results in the loss of any render data that was returned from the block's build method.

Note that there are two ways a block plugin can expose render data. Since block plugins implement CacheableDependencyInterface, they can declare their cache metadata via dedicated methods on the class. Layout builder correctly handles this metadata, even if the block is empty.

The other way cache metadata can be set on the returned render array from the block's build method. This is what layout builder is ignoring.

Steps to Reproduce

1. Check all caching is turned on
2. Create tiny module that has a block plugin that varies based on the existence of a query param. If the param is not present, the block should output nothing but the cache metadata. See below for code.
3. Add that block to layout of a page in layout builder
4. Visit the page as a logged out user. Block is not output (correct behavior)
5. Now visit the page and include the query string parameter that should force the block to output text. Observe that the block is still not output

Note that this affects the render cache as well as page/dynamic cache. Render cache is involved here because the rendered output of entity view displays is cached.

The code for the simple block:

namespace Drupal\simple_block\Plugin\Block;

use Drupal\Core\Block\BlockBase;

<?php
/**
 * @Block(
 *   id = "simple_block",
 *   admin_label = @Translation("Simple Block"),
 *   category = @Translation("Lists")
 * )
 */
class SimpleBlock extends BlockBase {
  public function build() {
    $build = [
      '#cache' => [
        'contexts' => ['url.query_args'],
      ],
    ];
    if (\Drupal::request()->query->has('test')) {
      $build['#markup'] = 'Test param found!';
    }
    return $build;
  }
}

This is a simple example, but node that ViewsBlock plugin behaves the same way. It outputs cache metadata in the render array.

I originally encountered this when I discovered that when Search API was indexing my nodes using the "rendered entity" field indexer, it was corrupting the render cache. Search API indexer ran on cron where there was no URL for my block to pull from, so the block returned empty render data. Layout builder didn't use the cache tags, so the cache was corrupted. Now when a visitor visits that page, the bad cache data is used and the block is empty.