Distinguish null result from error

[dvdplm]

Closes #117

When a remote endpoint replies with e.g. {"jsonrpc":"2.0","result":null,"id":1} we get a None from next_repsponse(). This is not an error so the current implementation is a bit confusing.
common::Response (and Serde) doesn't parse null as a valid JSON document (see https://stackoverflow.com/questions/8526995/is-null-valid-json-4-bytes-nothing-else) so this PR replaces null with [] which does parse. Not elegant but perhaps better than the status quo?

[tomusdrw]

It doesn't feel right: None probably just means that the stream is over, I don't see how null get's converted to None in that stream.

@svyatonik can you take a look? You have much better understanding of the codebase than I do.

[svyatonik]

I think Pierre has most of knowledge here :) But imo Tomek is right. My understanding is:StreamExt::next() returns None if Stream::poll_next() returns Ready(None). Usually it means that the stream is finished. But here we deal with FuturesUnordered, which may also return Ready(None) when it is empty (and it is empty initially, or when all futures have completed). We have had the similar problem in #107.

Afaiu, the most straightforward solution would be to replace FuturesUnordered with channel here - i.e. whenever response is received, background thread sends it to the channel. And the next_response is just waiting for the first response it sees. I may be wrong, or Pierre may suggest a better solution - just sharing my thoughts :)