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) then 1 (true) is returned.
The formula can also be called without arguments:
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.