Fix issues reported by PHPCS

MR Raised

Among the numerous meaningless changes for coding standards, the patch also fixes a deprecation warning:

 ( ! ) Deprecated: Drupal\computed_field\Plugin\ComputedField\ComputedFieldBase::createComputedFieldDefinition(): Implicitly marking parameter $bundle as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/web/modules/contrib/computed_field/src/Plugin/ComputedField/ComputedFieldBase.php on line 131

Unfortunately it breaks the module in a worse way:

PHP Fatal error:  Uncaught Error: Call to a member function getName() on null in /var/www/html/web/core/lib/Drupal/Core/Config/Entity/ConfigEntityStorage.php:188
Stack trace:
#0 /var/www/html/web/core/lib/Drupal/Core/Entity/EntityStorageBase.php(312): Drupal\Core\Config\Entity\ConfigEntityStorage->doLoadMultiple()
#1 /var/www/html/web/modules/contrib/computed_field/computed_field.module(122): Drupal\Core\Entity\EntityStorageBase->loadMultiple()
#2 /var/www/html/web/core/lib/Drupal/Core/Entity/EntityFieldManager.php(415): computed_field_entity_bundle_field_info()