LP:738091 - cast(timestamp() AS time returns NULL for 0000-00-00 00:00:00 in 5.1-micro

In 5.1-micro, the following expression

cast(timestamp(f1) AS time)

returns NULL if f1 = 0000-00-00 00:00:00 . In mysql-5.1 , it returns 00:00:00, which is the more intuitive result

test case:

drop table t1;
create table t1 (f1 datetime);
insert into t1 values ('0000-00-00 00:00:00');
select cast(timestamp(f1) AS time) from t1;