Allow XB to be gradually adopted: make XB's viewing (formatter) and editing (`ApiLayoutController::get()`) work with an empty XB field

Overview

Previously reported by … multiple people, but most recently by Gábor Hojtsy.

1. Install Standard install profile
2. Install experience_builder
3. Create article node — let's assume it's node 1
4. Install xb_dev_standard
5. Go to /node/1 and you'll get not the rendered article but … a fatal error:
   

   The website encountered an unexpected error. Try again later.
   
   AssertionError: assert($items->count() === 1) in assert() (line 30 of modules/contrib/experience_builder/src/Plugin/Field/FieldFormatter/NaiveComponentTreeFormatter.php).
   Drupal\experience_builder\Plugin\Field\FieldFormatter\NaiveComponentTreeFormatter->viewElements(Object, 'en') (Line: 91)
   Drupal\Core\Field\FormatterBase->view(Object, 'en') (Line: 275)
   Drupal\Core\Entity\Entity\EntityViewDisplay->buildMultiple(Array) (Line: 340)
   Drupal\Core\Entity\EntityViewBuilder->buildComponents(Array, Array, Array, 'full') (Line: 24)
   Drupal\node\NodeViewBuilder->buildComponents(Array, Array, Array, 'full') (Line: 282)
   Drupal\Core\Entity\EntityViewBuilder->buildMultiple(Array) (Line: 239)
   …
   Drupal\Core\DrupalKernel->handle(Object) (Line: 21)
   

6. Try to edit this article in XB, by navigating to /xb/node/1/editor, which will not lead to a working XB UI, but to:
   

Both of these make trying XB, or gradually adopting it a very unfriendly (and even frightening) experience on existing content.

Proposed resolution

The crash in point 5 is caused by \Drupal\experience_builder\Plugin\Field\FieldFormatter\NaiveComponentTreeFormatter::viewElements() requiring a non-empty XB field.

The crash in point 6 is caused by \Drupal\experience_builder\Controller\ApiLayoutController::get() having a similar assumption.

User interface changes

No more crashes.