NULL("FieldName");

The formula can be called with 1 argument.

In this case it will check the value of the field and return 1 (true) if the value is NULL, or otherwise return 0 (False).

Example

NULL("Address")
If the field "Address" has no value (is Null) the 1 (true) is returned.

The formula can also be called without arguments:

NULL();

The result of this formula is a Null value. When you map that into a field in the destination database, the field will be set to NULL in the destination database.