SET SERVEROUTPUT ON
DECLARE
  PROCEDURE test_json (p_json IN CLOB) IS
  BEGIN
    IF p_json
IS JSON
THEN
      DBMS_OUTPUT.put_line(p_json || ' - IS JSON');
      IF
JSON_EXISTS(p_json, '$.employee_no' FALSE ON ERROR)
THEN
        DBMS_OUTPUT.put_line('employee_no = ' ||
JSON_VALUE(p_json, '$.employee_no')
);
        DBMS_OUTPUT.put_line('employee_no = ' ||
JSON_QUERY(p_json, '$.employee_no' WITH CONDITIONAL WRAPPER)
);
      END IF;
    ELSE
      DBMS_OUTPUT.put_line(p_json || ' - IS NOT JSON');
    END IF;
  END;
BEGIN
  test_json('banana');
  test_json('{ "employee_no":9999 }');
END;
/
banana - IS NOT JSON
{ "employee_no":9999 } - IS JSON
employee_no = 9999
employee_no = [9999]

PL/SQL procedure successfully completed.

SQL>