Implementation of abstract method FieldItemBase::schema cannot return empty array

I’m trying to create a truly computed field having no schema.

As stated in API documentation the schema() implementation should return an empty array, but trying this resolves in the following error when I add the computed field to a node type:

Drupal\Core\Database\DatabaseExceptionWrapper: SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ') LIMIT 1 OFFSET 0' at line 1: SELECT 1 AS expression FROM {node_revision__field_wwsww} t WHERE () LIMIT 1 OFFSET 0; Array ( ) in Drupal\Core\Entity\Sql\SqlContentEntityStorage->countFieldData() (line 1679 of C:\drupal8\core\lib\Drupal\Core\Entity\Sql\SqlContentEntityStorage.php).SeverityErrorHostname::1Operations

Here is the example code I’m using:

namespace Drupal\mytestfield\Plugin\Field\FieldType;

use Drupal\Core\Field\FieldItemBase;
use Drupal\Core\Field\FieldStorageDefinitionInterface;
use Drupal\Core\TypedData\DataDefinition;

/**
 * Plugin implementation of the 'field_gpxElevation' field type.
 *
 * @FieldType(
 *   id = "field_gpxElevation",
 *   label = @Translation("GPX Elevation Field"),
 *   module = "gpx_track_elevation_field",
 *   description = @Translation("Demonstrates a field composed of an RGB color."),
 *   default_widget = "gpx_track_elevation_field_text",
 *   default_formatter = "gpx_track_elevation_field_simple_text"
 * )
 */
class myTestField extends FieldItemBase {
  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return array();
  }

  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties['processed'] = DataDefinition::create('string')
      ->setLabel(t('My test Field'))
      ->setDescription(t('The computed text value.'))
      ->setComputed(TRUE)
      ->setClass('\Drupal\mytestfield\Plugin\myTestFieldProcessor');

    return $properties;
  }

}

Can anyone help me understanding how to create the schemaless field?

Thank you