1. Render regions with no children, so empty checks work

Problem/Motivation

The simplest case for #953034: [meta] Themes improperly check renderable arrays when determining visibility is a region that just consists of [#cache] but no renderable children.

In that case the region MUST be rendered, so even if in a twig template we did:

{% if (element_is_empty(content.sidebar_left)) %}
<div class="sidebar-left">
  {{ content.sidebar_left }}
</div>
{% endif %}

in fact it would _need_ to be:

{% if (element_is_empty(content.sidebar_left)) %}
<div class="sidebar-left">
  {{ content.sidebar_left }}
</div>
{% else %}
  {{ content.sidebar_left }}
{% endif %}

which is very un-intuitive OR element_is_empty would need to actually render the content, which is less than ideal due to performance reasons as we would need to render the sidebar always twice.

Proposed resolution

In BlockPageVariant render empty regions.

Remaining tasks

- Discuss

User interface changes

API changes

Data model changes