Sorry, I'm not sure about this I use SQL Passthrough.
I'm using Micosoft ODBC driver for Oracle. Which I find pretty good.

I usually do something like..

col1val=""
lcsql="update test set col1=?col1val"
SQLEXEC(h,lcsql)

When I do this it sets the value to null. But the field must be
varchar2 (i don't know what a char type would do.)

Maybe you can just set the values to NULL...Instead of "" like I mentioned.

I don't know about any settings that handle this.