Don't send back empty proofs if light request fails

[tomaka]

At the moment, if the node receives a so-called "light client request" (asking for a storage proof or a call proof), and this request cannot be served, we send back a message indicating that everything is ok, except that it contains an empty proof.
The reason why a request cannot be served is typically that the block has been pruned.

This PR modifies the protocol so that we send back a message without any proof instead.

To do an analogy, right now we more or less send back Some(Vec::new()), while this PR modifies this to send back None.

This makes it clearer from the point of view of the request sender that something bad has happened.

[cheme]

Change looks good (til I learn that oneof is not required).
I am wondering a bit about versioning, but will probably leave it like this.
Would need to be mentioned in release note as potentially breaking through.

[tomaka]

I am wondering a bit about versioning, but will probably leave it like this.

As far as I know, only smoldot sends these requests at the moment, and I'm ok with my own change.

[lamafab]

What about the Header and Changes responses (currently both still just return HandleRequestError::BadRequest("Not supported."))?

EDIT: Changes tries were deprecated, so I guess the Changes request/response messages will be removed anyway.

[bkchr]

bot merge