考虑自定义一个函数，例如

SQL> l
CREATE OR REPLACE FUNCTION custom_jsonb_build_object(key varchar2, value varchar2)
return jsonb
AS
BEGIN
IF value IS NULL THEN
    RETURN jsonb_build_object(key, '');
ELSE
    RETURN jsonb_build_object(key, value);
END IF;
END;

SQL> /
操作已执行
已用时间: 25.207(毫秒). 执行号:1315.
SQL> select custom_jsonb_build_object('c1', null) from dual;

行号     CUSTOM_JSONB_BUILD_OBJECT(:<1>,NULL)
---------- ------------------------------------
1          {"c1":""}

已用时间: 5.241(毫秒). 执行号:1316.

SQL> select jsonb_build_object('c1', null) from dual;

行号     JSONB_BUILD_OBJECT(:<1>,NULL)
---------- -----------------------------
1          {"c1":null}