Using LEFT JOIN could achieve the goal.
SELECT e FROM Event e LEFT JOIN e.rule r ORDER BY r.rowNum ASC
which will generate the following SQL:
SELECT t0.uuid, t0.eventTime, t2.uuid, t2.rowNum, t1.rowNum FROM Event t0 LEFT OUTER JOIN Rule t1 ON t0.RULE_UUID = t1.uuid LEFT OUTER JOIN Rule t2 ON t0.RULE_UUID = t2.uuid ORDER BY t1.rowNum ASC
There is something in the JPQL need to pay attention: the ORDER BY must be applied to r.rowNum rather than e.rule.rowNum, otherwise, it will generate an INNER JOIN.
You may have noticed that the generated SQL have two OUTER JOINs on RULE table.