You can use a not equal (<>) functoid, and a value mapping functoid.
- Drag a line from the source element or attribute to the <> functoid.
- Create a constant in this functoid, but leave it blank.
- Drag a line from there to the value mapping functoid, as it needs to use the logical as its first connection.
- Now connect the same source element or attribute to the value mapper.
- Then from there to the output schema element.
- (And be sure the output element is nilable/optional.)
Have you tried this? Let me know if it doesn't work as expected and we can check a few properties.
This does assume you have 2 schemas. There are other ways to accomplish this, but this, I believe, makes the most sense.