rust: kunit: use `pin_init::zeroed` instead of custom null value

rust: kunit: use `pin_init::zeroed` instead of custom null value

The last null element can be created (constly) using `pin_init::zeroed`,
so prefer to use it instead of adding a custom way of building it.

Reviewed-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: David Gow <davidgow@google.com>
Link: https://patch.msgid.link/20260112170919.1888584-12-gary@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>