Check Xpath result not empty

I have a XML file and I want to use xpath to map the data to a new object. So far I can map every field, but now I want a if-else construction. e.g: If element 1 from question 3.1 does not contain data, use element 2 from question 3.2 and put it in the target object

Below an example of the XML and the mapping I use. In the example both ElementID='1' and ElementID='2' is mapped to targetField, but here I want to use the if-else. Can anyone help me out with this?

Mapping: mapping example

XML: data