We can look at the last bit of the principle as a unit. I would think of it as whether or not the theory is falsifiable. We could diagram the initial principle as
If not falsifiable then not empirical.
We should also note down that If not refutable, then not falsifiable.
I think this is a case where it is better to have a strong understanding of what the passage is saying than to be able to formally diagram all the conditional logic. The principle sets up a necessary condition- that a theory must be able to be proven false. Just because it uses an If statement in that necessary condition does not mean we need to overcomplicate the statement.