Stop removing `nil` messages in stream responses
Stop removing `nil` messages in stream responses

I was the original author of this change some time ago, and I now think it is a mistake. If redis decides to return `nil` values then I don't think `aioredis` should eat them.

In my case I am receiving `nil` values when performing an `XCLAIM`. Why does this happen? I'm not sure*, but I know that I definitely want to be able to `ACK` these `nil` messages in order to stop my consumer continually reclaiming them.

Removing of `nil` values by `aioredis` means the response I get looks like, "you could not claim this message," not the more accurate, "you claimed this message but there is no body".

* Perhaps they have been trimmed from the end of the stream but redis still sees them as claimed, so just returns `nil` rather than the message body.
aio-libs/aioredis,aio-libs/aioredis