feat: #3536245 Expose views empty area as a slot on the custom element

Ciprian Stavoveirequested to merge
issue/lupus_decoupled-3536245:3536245-views-add-support into 1.x

When a Drupal View returns no results, the configured "No results behavior" (empty area) is now exposed as an 'empty' slot on the custom element. Each area handler's render array is converted via createFromRenderArray(), so entity-rendering plugins produce structured custom elements while plain text/HTML falls back to drupal-markup.

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

Closes #3536245

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