rust: pin-init: implement `ZeroableOption` for `&T` and `&mut T`

`Option<&T>` and `Option<&mut T>` are documented [1] to have the `None`
variant be all zeroes.

Link: https://doc.rust-lang.org/stable/std/option/index.html#representation [1]
Link: https://github.com/Rust-for-Linux/pin-init/pull/56/commits/5ef1638c79e019d3dc0c62db5905601644c2e60a
Link: https://lore.kernel.org/all/20250523145125.523275-5-lossin@kernel.org


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