When creating the interface for loading Cost Center rows, I couldn't load null value

Summary:

When creating the interface for loading Cost Center rows, I couldn't use HDL because it's not a supported Business Object.

 

Moreover I can't insert #NULL in the mapping when I want to get a null value in the database, as is in the case of import using Data Loader. 

Loading data in the interface is done using SOAP. How do I insert null into a database? Which sign should I upload for this purpose? 

When I send #NULL I receive a string value in the Description database field: #NULL, and I'd like to have an empty cell in the Description field.