Oracle 18c

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CREATE TABLE test_data ( value CLOB );
INSERT INTO test_data ( value ) VALUES ( '{"id": "33", "type": "abc", "val": "2", "cod": "", "sg1": "1", "sg2": "1"}' );
1 rows affected

INSERT INTO test_data ( value ) VALUES ( '{"id": "359", "type": "abcef", "val": "52", "cod": "aa", "sg1": "", "sg2": "0"}' );
1 rows affected

SELECT JSON_OBJECT(
         'id'   IS j.id,
         'type' IS j.typ,
         'val'  IS j.val,
         'cod'  IS j.cod,
         'sg1'  IS j.sg1,
         'sg2'  IS j.sg2
       ) AS JSON
FROM   test_data t
       CROSS JOIN
       JSON_TABLE(
         t.value,
         '$'
         COLUMNS
           id  NUMBER(5,0) PATH '$.id',
           typ VARCHAR2(10) PATH '$.type',
           val NUMBER(5,0) PATH '$.val',
           cod VARCHAR2(10) PATH '$.cod',
           sg1 NUMBER(5,0) PATH '$.sg1',
           sg2 NUMBER(5,0) PATH '$.sg2'
       ) j


JSON


{"id":33,"type":"abc","val":2,"cod":null,"sg1":1,"sg2":1}

{"id":359,"type":"abcef","val":52,"cod":"aa","sg1":null,"sg2":0}


SELECT REGEXP_REPLACE(
         REGEXP_REPLACE(
           value,
           '"(id|val|sg1|sg2)": ""',
           '"\1": "null"'
         ),
         '"(id|val|sg1|sg2)": "(\d+|null)"',
         '"\1": \2'
       ) AS JSON
FROM   test_data


JSON


{"id": 33, "type": "abc", "val": 2, "cod": "", "sg1": 1, "sg2": 1}

{"id": 359, "type": "abcef", "val": 52, "cod": "aa", "sg1": null, "sg2": 0}