Apparently oracle doesn\'t seem to distinguish between empty strings and nulls. E.g.
Select name from TABLE_A where id=100;
ID NAME
100 null
Update
Looks like Oracle have said that this behaviour may be change in a future release. When and which release it will be is not mentioned.
If you have access to metalink look at note: 1011340.6 (unfortunately because of restrictions I am unable to copy the contents of the note here)
If you don't have access to metalink then look look at the following from the 10g release 2 documentation here