rusticl/api: Interpret `lengths` as a slice of Option<NonZeroUsize>

rusticl/api: Interpret `lengths` as a slice of Option<NonZeroUsize>

So far `lengths` as been interpreted as a slice of usize. However, 0 is specified as a sentinel
value signalling that the corresponding string is nul terminated. Since checking for sentinel
values is frequently forgotten, Option types should be preferred if possible.

Option<NonZeroUsize> is layout compatible with usize. The None variant is then represented as 0,
which is exaclty what we need here.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18978>