If you can't put a NOT NULL constraint on there, then what might work is a function-based index .

You could create an index like ...

create index my_index on log_record (NVL(TRAN_DATETIME,TO_DATE('01-JAN-3000','DD-MON-YYYY')));

Then the clause would be ...

ORDER BY NVL(TRAN_DATETIME,TO_DATE('01-JAN-3000','DD-MON-YYYY'))

... and you would adjust the '01-JAN-3000' date according to whether you want the nulls at the end or the beginning.