rust: pin-init: change `impl Zeroable for Option<NonNull<T>>` to `ZeroableOption for NonNull<T>`

This brings it in line with references. It too is listed in [1].

Link: https://doc.rust-lang.org/stable/std/option/index.html#representation
Link: https://github.com/Rust-for-Linux/pin-init/pull/56/commits/8e52bf56ddc2190ce901d2f7c008ab8a64f653a9
Link: https://lore.kernel.org/all/20250523145125.523275-6-lossin@kernel.org


Signed-off-by: Benno Lossin <lossin@kernel.org>