Null Value for Export Tasks
By default, when an export file is created, Null Value fields are replaced with a space, unless the output file format is VARIABLE, when it replaces them with the explicit "NULL" string.
When running a job, you can use the nvl parameter to define a Null Value Fill (NVL) character, which will override the default, as follows:
The first example changes the NVL character to "#" in the specified external set. The second example changes it for the entire job.