coalesce nil UUID for null agreement

backend/modules/core/data/event.js

@@ -53,12 +53,12 @@ async function getEvent(client, userId, id, includeData, meta) {
                 'senderId', e.sender_id,
                 'recipientId', e.recipient_id,
                 'created', e.created,
-                'agreementId', (
+                'agreementId', COALESCE((
                     SELECT a.agreement_id_uuid
                     FROM agreement a
                     WHERE a.id = e.agreement_id
                     AND a.user_id = $1
-                ),
+                ), '00000000-0000-0000-0000-000000000000'),
                 ${dataSql}
                 'created', e.created
             ) AS record