In order to export configuration entities to configuration we map their properties to configuration object keys. The mapping is obtained from the configuration entity type config_export property. If this is not present we use the configuration entity's schema is it available. In Drupal 8.5 this functionality is spread between ConfigEntityBase::toArray() and ConfigEntityType::getPropertiesToExport(). In Drupal 8.6 all of the functionality is present in ConfigEntityType::getPropertiesToExport() which now optionally is passed the configuration entity ID. Without the ID we cannot fall back to schema.
It is recommended that all configuration entity types have a config_export property in their annotation.
Config schema for configuration entities should still exist though.
