Example "opty" fist instance has to be empty string and the second one has to be null
Here how you want the map to look like
To crate first mapping, click on optyId field to take you into Mapping Builder.
Then open Mapping Components on the left bottom panel and drag and drop "value-of" on top of optyId (or in the "-- Drag and Drop or Type value ...'." empty child row)
In the child empty row of the "select" attribute type quote quote (""). Save and close
For the second map to be null, first we need to repeat optyId. Use Repeat element from the context menu (right click)
Now, click on optyId (2 of 2) to get into Mapping Builder.
From Mapping components drag attribute name='xsi:nil' and drop it on top of optyId
Save and close
There you have it!