I've solved this issue this way:


    function hook_entity_base_field_info_alter(&$fields, EntityTypeInterface $entity_type) {
      if ($entity_type->id() == 'contact_message') {
        if (isset($fields['message']) && $fields['message'] instanceof BaseFieldDefinition) {
          /* @var BaseFieldDefinition $data */
          $data = &$fields['message'];
          $data->setRequired(FALSE);
        }
      }
    }