Fix nullability detection for some arguments

Specifically, `find_route`'s `first_hops` argument should be a nullable-ish pointer. See comment:

> Note that first_hops (or a relevant inner pointer) may be NULL or all-0s to represent None

Start at the parser logic for the `find_route` function and its `first_hops` argument, then trace how nullability is detected for pointer arguments. Use the issue's comment as the expected behavior: NULL or all-zero `first_hops` (or a relevant inner pointer) should represent None; done when that case is classified as nullable-ish.

Written by the indexing model from the issue text.