qom: Constify TypeInfo::class_data

All callers now correctly expect a const class data.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250424194905.82506-5-philmd@linaro.org>

rust/qemu-api/src/qom.rs

@@ -513,7 +513,7 @@ pub trait ObjectImpl: ObjectType + IsA<Object> {
         class_size: core::mem::size_of::<Self::Class>(),
         class_init: Some(rust_class_init::<Self>),
         class_base_init: Self::CLASS_BASE_INIT,
-        class_data: core::ptr::null_mut(),
+        class_data: core::ptr::null(),
         interfaces: core::ptr::null_mut(),
     };