Detect if an aggregate is null in a SQL case conditional statement - postgresql

Question

I have 3 tables:
entities: list of entities, some of them can participate to an event;
events: list of the events;
participations: list of participations
Here are the structures:
ENTITIES
id | name
EVENTS
id | name | date_start | date_end
PARTICIPATIONS
id | id_event | id_participation | type
I'd like to get all the entities, with in the column "events" the array of all the events. Here is what gives me a good result:
SELECT
e.id,
e.name,
array_agg(
json_build_object(
'id', evts.id,
'name', evts.name,
'date_start', evts.date_start,
'date_end', evts.date_end
)
) as events
FROM g.entities e
LEFT JOIN public.events_participations p
ON (
p.id_participation::integer = e.id
AND type ='entity'
)
LEFT JOIN public.events evts
ON evts.id = p.id_event
WHERE e.id = 22
GROUP BY p.id_participation, e.id
BUT (and there is a but otherwise I wouldn't write here) when an entity doesn't participate in any event, I have this in the column:
{"{\"id\" : null, \"name\" : null, \"date_start\" : null, \"date_end\" : null}"}
I thought I could get a "NULL" because of the LEFT JOIN.
I tried to use "CASE" to detect when there are no event, but it doesn't seem to be the solution. How could I get the expected result and get an array only if there is at least one event and NULL otherwise?
Thanks in advance.
EDIT:
I tried this:
WITH list_events AS (
SELECT
e.id,
json_build_object(
'id', evts.id,
'name', evts.name,
'date_start', evts.date_start,
'date_end', evts.date_end
) as events
FROM g.entities e
LEFT JOIN public.events_participations p
ON (
p.id_participation::integer = e.id
AND type ='entity'
)
JOIN public.events evts
ON evts.id = p.id_event
WHERE e.id = 1
)
SELECT
e.id,
e.nom,
array_agg(
l.*
) as events
FROM g.entities e
LEFT JOIN list_events l
ON e.id = l.id
WHERE e.id = 1
GROUP BY e.id
But instead of having "NULL" I have en empty column of type "record[]".

Answer 1

I was almost there:
WITH list_events AS (
SELECT
e.id,
json_build_object(
'id', evts.id,
'name', evts.name,
'date_start', evts.date_start,
'date_end', evts.date_end
) as events
FROM g.entities e
LEFT JOIN public.events_participations p
ON (
p.id_participation::integer = e.id
AND type ='entity'
)
JOIN public.events evts
ON evts.id = p.id_event
WHERE e.id = 1
)
SELECT
e.id,
e.nom,
array_agg(
l.events
) as events
FROM g.entities e
LEFT JOIN list_events l
ON e.id = l.id
WHERE e.id = 1
GROUP BY e.id