Add example use empty slices.

[ava57r]

Need more examples how use empty slice.

fn main() {
    let vec = vec![1, 2];
   println!("{:?}", test(&vec).iter().next());
}

fn test(vec:&Vec<u32>) -> &[u32] {
if vec.len() == 3 {
  vec.as_slice()
} else {
  &[]
}
}