add default empty state for Sample structure

[milyin]

Describe the release item

The Rust API provides two variants of mutable access to Sample in reply: result_mut and into_result:

pub struct Reply {
    pub(crate) result: Result<Sample, ReplyError>,
...
}
impl Reply {
    pub fn result_mut(&mut self) -> Result<&mut Sample, &mut ReplyError> {...}
    pub fn into_result(self) -> Result<Sample, ReplyError> {...}
}

In zenoh-c API the only way to take ownership on Sample is through &mut Sample, but it's impossible unless we can replace sample to valid empty one.

E.g. this update eclipse-zenoh/zenoh-c#718 is actually blocked because we can't safely provide take operation from mutable sample reference.