Field should not be rendered when DsField plugin only returns cache tags

Problem/Motivation

If you have a DsField plugin that calculates the contents of its result you must always include the cacheable metadata. Returning FALSE will cause your plugin to be forever cached as empty.

Example:

$build = [];
$cache = CacheableMetadata::createFromRenderArray($build)->addCacheableDependency($obj);
$cache->applyTo($build);
if ($obj->doSomething()) {
  $build['#markup'] = 'something here');
}

return $build;

In this instance where doSomething() returned FALSE, then you won't have anything to actually render, only some cache tags, which might look like:

    $build = [
      '#cache' => [
        'tags' => ['obj:1'],
      ],
    ];

In ds_entity_view_alter we do if (!empty($field_value)) { and then add our #theme field as a wrapper. This is added even when all that was returned was cache data and the field output should have been empty. Furthermore, if you do return FALSE from your plugin, you don't have the chance to add any cache data.

Proposed resolution

I can see a couple of different approaches. We could do something pretty gross like this:

if (!empty(Element::children($field_value)) && !isset($field_value['#type']) && !isset($field_value['#theme']) && !isset($field_value['#markup'])) {

Or, we can go ahead and track the cache data separately, which would mean that we pass in a BubbleableMetadata object.

$cache = BubbleableMetadata::createFromRenderArray($build);
$field_value = $field_instance->build($cache);

Then we can continue to check !empty($field_value) but always apply the cache data later on.

API changes

I think we can avoid any API breaks if we make the $cache parameter to build() optional and then merge in any cache data that is also returned to $field_value for others that were already doing that.