Improve empty stream responses deserialization

[godric]

This is to fix an error that still happens despite #905:

When attempting to read trimmed, pending messages for a consumer group from a stream, we still get

NoMethodError: undefined method each_slice' for nil:NilClass

This is because Redis keeps the entry_id despite the entry itself having been deleted, and will respond with, eg:

% redis-cli XREADGROUP GROUP test_group test_consumer STREAMS test_key 0
1) 1) "test_key"
   2) 1) 1) "1627563001384-0"
         2) (nil)

The branch provides a spec to reproduces the issue and validate the fix. Let me know if you need me to do changes.
And thanks for this great gem 👍

PS: This is related to the bug mentioned in issues #936, #929. Although I wasn't able to reproduce the issue with XCLAIM, potentially thanks to the previous fix in #905

[byroot]

The CI failures look legit.

[godric]

Sorry, I utterly failed my attempt to rebase on redis-rb/master
I also realised one of my assertion was wrong, and fixed it, it should pass the CI now.

[byroot]

Redis::CommandError: ERR unknown command `xgroup`, with args beginning with: `create`, `k1`, `g1`, `0`, `MKSTREAM`,

You need to handle older redises in the test suite.

[byroot]

This should go in https://github.com/redis/redis-rb/blob/master/test/lint/streams.rb (that will give you the automatic skip if redis is too old)

[godric]

I renamed the test and moved it with other xreadgroup tests ✅

[byroot]

Thank you.