Handle empty stream responses

[byroot]

Fix: #905
Ref: #906

See the provided repro script I wasn't able to convert it into a test case as this empty response is only returned

call: redis.xgroup(:create, "58h4o7g", "a", "$", {:mkstream=>true})
done: redis.xgroup(:create, "58h4o7g", "a", "$", {:mkstream=>true})
call: redis.xadd("58h4o7g", {:test=>"0"})
done: redis.xadd("58h4o7g", {:test=>"0"})
call: redis.xadd("58h4o7g", {:test=>"1"})
done: redis.xadd("58h4o7g", {:test=>"1"})
call: redis.xadd("58h4o7g", {:test=>"2"})
done: redis.xadd("58h4o7g", {:test=>"2"})
call: redis.xadd("58h4o7g", {:test=>"3"})
done: redis.xadd("58h4o7g", {:test=>"3"})
call: redis.xadd("58h4o7g", {:test=>"4"})
done: redis.xadd("58h4o7g", {:test=>"4"})
call: redis.xadd("58h4o7g", {:test=>"5"})
done: redis.xadd("58h4o7g", {:test=>"5"})
call: redis.xadd("58h4o7g", {:test=>"6"})
done: redis.xadd("58h4o7g", {:test=>"6"})
call: redis.xadd("58h4o7g", {:test=>"7"})
done: redis.xadd("58h4o7g", {:test=>"7"})
call: redis.xadd("58h4o7g", {:test=>"8"})
done: redis.xadd("58h4o7g", {:test=>"8"})
call: redis.xadd("58h4o7g", {:test=>"9"})
done: redis.xadd("58h4o7g", {:test=>"9"})
call: redis2.xreadgroup(:a, 8, "58h4o7g", ">", {:count=>8})
call: redis3.multi()
call: redis3.xack("58h4o7g", :c, "1592564769910-1")
done: redis3.xack("58h4o7g", :c, "1592564769910-1")
call: redis3.xdel("58h4o7g", "1592564769910-1")
done: redis3.xdel("58h4o7g", "1592564769910-1")
call: redis4.xclaim("58h4o7g", :a, :a, 8, "1592564769909-2")
done: redis2.xreadgroup(:a, 8, "58h4o7g", ">", {:count=>8})
call: redis2.xreadgroup(:a, 4, "58h4o7g", ">", {:count=>1})
done: redis3.multi()
done: redis2.xreadgroup(:a, 4, "58h4o7g", ">", {:count=>1})
call: redis3.multi()
call: redis3.xack("58h4o7g", :c, "1592564769909-0")
done: redis3.xack("58h4o7g", :c, "1592564769909-0")
call: redis3.xdel("58h4o7g", "1592564769909-0")
done: redis3.xdel("58h4o7g", "1592564769909-0")
done: redis4.xclaim("58h4o7g", :a, :a, 8, "1592564769909-2")
done: redis3.multi()
call: redis4.xclaim("58h4o7g", :a, :b, 4, "1592564769911-1")
call: redis2.xreadgroup(:a, 8, "58h4o7g", ">", {:count=>8})
done: redis4.xclaim("58h4o7g", :a, :b, 4, "1592564769911-1")
done: redis2.xreadgroup(:a, 8, "58h4o7g", ">", {:count=>8})
call: redis4.xclaim("58h4o7g", :a, :a, 8, "1592564769910-2")
call: redis2.xreadgroup(:a, 1, "58h4o7g", ">", {:count=>1})
call: redis3.multi()
call: redis3.xack("58h4o7g", :c, "1592564769910-1")
done: redis3.xack("58h4o7g", :c, "1592564769910-1")
call: redis3.xdel("58h4o7g", "1592564769910-1")
done: redis3.xdel("58h4o7g", "1592564769910-1")
done: redis2.xreadgroup(:a, 1, "58h4o7g", ">", {:count=>1})
done: redis4.xclaim("58h4o7g", :a, :a, 8, "1592564769910-2")
call: redis2.xreadgroup(:a, 4, "58h4o7g", ">", {:count=>4})
done: redis3.multi()
call: redis4.xclaim("58h4o7g", :a, :b, 8, "1592564769910-0")
done: redis2.xreadgroup(:a, 4, "58h4o7g", ">", {:count=>4})
call: redis3.multi()
call: redis3.xack("58h4o7g", :c, "1592564769909-0")
done: redis3.xack("58h4o7g", :c, "1592564769909-0")
call: redis2.xreadgroup(:a, 4, "58h4o7g", ">", {:count=>1})
done: redis4.xclaim("58h4o7g", :a, :b, 8, "1592564769910-0")
call: redis3.xdel("58h4o7g", "1592564769909-0")
done: redis3.xdel("58h4o7g", "1592564769909-0")
call: redis4.xclaim("58h4o7g", :a, :b, 1, "1592564769911-0")
done: redis3.multi()
done: redis2.xreadgroup(:a, 4, "58h4o7g", ">", {:count=>1})
call: redis3.multi()
call: redis3.xack("58h4o7g", :a, "1592564769911-2")
done: redis3.xack("58h4o7g", :a, "1592564769911-2")
done: redis4.xclaim("58h4o7g", :a, :b, 1, "1592564769911-0")
call: redis2.xreadgroup(:a, 1, "58h4o7g", ">", {:count=>4})
call: redis3.xdel("58h4o7g", "1592564769911-2")
done: redis3.xdel("58h4o7g", "1592564769911-2")
done: redis2.xreadgroup(:a, 1, "58h4o7g", ">", {:count=>4})
done: redis3.multi()
call: redis4.xclaim("58h4o7g", :a, :b, 1, "1592564769909-0")
call: redis3.multi()
call: redis2.xreadgroup(:a, 1, "58h4o7g", ">", {:count=>8})
done: redis4.xclaim("58h4o7g", :a, :b, 1, "1592564769909-0") # get [nil] and fails

If you replay the same scenario but serially, that last xclaim will receive and empty array ([]) rather an array with a single nil entry ([nil]).

I couldn't locate this behavior anywhere in the doc, I can't say for sure it's a bug, but neither can I say it's a deliberate feature. I can only assume xclaim response is affected by another in flight command.

The original repo was done on Redis 5, but I was able to repo with Redis 6.0.5