Answer based on @dale-k's comment:

ORDER BY
  CASE WHEN :sortField = 'creationDate' AND :sortDirection = 'DESC' THEN update_.creation_date END DESC,
  CASE WHEN :sortField = 'creationDate' AND :sortDirection = 'ASC' THEN update_.creation_date END ASC,
  CASE WHEN :sortField is NULL THEN update_.default_time_field END DESC;