SwiperFormatter entity throws TypeError when created programmatically without a description

Problem/Motivation

The SwiperFormatter entity's $description property is typed as string, which does not accept null. However, the description field is optional in the admin UI form. This creates an inconsistency:

• Creating an entity via the UI without a description works fine (the form submits an empty string)
• Creating an entity programmatically without setting description fails with an error: TypeError: Cannot assign null to property Drupal\swiper_formatter\Entity\SwiperFormatter::$description of type string

Steps to reproduce

$entity = \Drupal::entityTypeManager()
  ->getStorage('swiper_formatter')
  ->create([
    'id' => 'test_swiper',
    'label' => 'Test Swiper',
  ]);
$entity->save();
// TypeError: Cannot assign null to property ... ::$description of type string

Proposed resolution

Initialize the $description property with an empty string default value on the entity class.
This makes the programmatic behavior consistent with the UI - description remains optional in both contexts, and the property type stays strict without needing to introduce nullability.

Remaining tasks

• Update the $description property default on SwiperFormatter
• Add a test to verify entity creation without a description (maybe?)
• Update existing tests to remove the descriptions that were added only because of the TypeError