Oracle 21c

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CREATE TABLE table_name (value) AS
SELECT CAST(NULL AS NUMBER) FROM DUAL;
1 rows affected

SELECT JSON_OBJECT(
         'field' VALUE JSON_ARRAYAGG(
                         JSON_OBJECT(
                           '@type'  VALUE 'idtype',
                           '@value' VALUE value
                           RETURNING BLOB
                         )
                         ABSENT ON NULL
                         RETURNING BLOB
                       )
                       ABSENT ON NULL
          RETURNING BLOB
        ) AS json
FROM    table_name
WHERE   value IS NOT NULL


JSON


{}


INSERT INTO table_name (value) VALUES (1);
1 rows affected

SELECT JSON_OBJECT(
         'field' VALUE JSON_ARRAYAGG(
                         JSON_OBJECT(
                           '@type'  VALUE 'idtype',
                           '@value' VALUE value
                         )
                         ABSENT ON NULL
                       )
                       ABSENT ON NULL
          RETURNING BLOB
        ) AS json
FROM    table_name
WHERE   value IS NOT NULL


JSON


{"field":[{"@type":"idtype","@value":1}]}