Retrieve Nulls element indexes in order to insert them in another list with the same indexes

Hey all,
I read some topics on how to use [item index] or [member index] but I still don’ t manage to solve this issue.
I have a set of curves as in figure

the first list is a set containing some nulls. Doing an offset of the curves it just takes the real curves and in the end i don’ have the nulls anymore, but just the offsetted curves.
What I need is to have the nulls in the same index positions after the offsetting, because i need the curves to match in number and positions before and after the offset.
I was thinking to use item index+ insert item but as I read in the other topics if I type “null” as element of search is not working. I got that this is not working cause the two nulls are not the same…

How can i Solve this issue?
mind that the number and the position of the nulls are variable so what I need is to keep is the pattern curves-nulls.

I upload a simple file with the same issue just simplified cause the original file is huge.
(I would find a way to keep the pattern rather than than re-applying all the 0 “manually”, as before the offset)

Any advices?
Thanks (5.7 KB) (2.8 KB)

Maybe this will help. Only works if input C is a flat list tho. Just a quick script…

like that?