Fix impl-serde::serializa/_raw for empty slices

[bkchr]

Fixes: paritytech/substrate#3946

[tomusdrw]

This is really frustrating. What happened to the entire suite of tests for this stuff? Was it not ported from the ethereum codebase?

[ordian]

We have some tests in https://github.com/paritytech/parity-common/blob/master/ethereum-types/tests/serde.rs. I think it was ported from https://github.com/paritytech/primitives/, don't see any additional tests there.

[niklasad1]

@tomusdrw The serde tests for ethereum-types were ported to this repo but I'm not sure how the tests differ.

As I said before, ethereum doesn't use serialize/deserialize Bytes from this repo so definitely not ported from ethereum but we should merge them ideally

EDIT: ethereum Bytes is using rustc::ToHex which was probably removed because of more allocations were required but I wasn't involved so I don't remember

[dvdplm]

It's been a while but I don't think there were serde tests in primitives, or only very basic.

[tomusdrw]

Indeed seems that tests for Bytes were missing, good that we are now getting some :)