SQL & PL/SQL

How set date to null if its corresponding column time is null

CREATE TABLE test
(C_DATE AS DATE
C_TIME AS VARCHAR 2);

INSERT INTO test VALUES('15/01/2001,'18:10');
INSERT INTO test VALUES('20/01/2001,'22:10');
INSERT INTO test VALUES('','');
INSERT INTO test VALUES('','13:00');
INSERT INTO test VALUES('','11:00');

 select c_date, c_time,
              case 
                when (c_date is null and c_time is not null) then c_ars_time IS NULL  
               end 
               from test;

currently the case statement gets ora-00905: missing keyword. any ideas would be appreciated.