[5.6] Fix check on empty queue

[pdavide]

When using phpredis, blpop returns an empty array if no elements are found in the list.
This can be easily deduced looking at this test.

Fix #23756.

[tillkruss]

I'd rather see the PhpRedisConnection fixed, so we can rely on the return values across drivers.

[pdavide]

Well, I think my simple solution will ensure compatibility across the drivers, as empty return true with both NULL and Array().
Additionally this little bug cause the application log file to grow fast and become unreadable because the queue worker is calling the blockingPop function every time it searches for an available job and the $rawBody[1] expression will raise an error when $rawbody is an empty array.

[tillkruss]

@pdavide: It's only solved in RedisQueue, nowhere else. See #23761.