[5.7] Added default array value for redis config

[robbytaylor]

If the redis config is missing from the database configuration then there's an error due to the first argument to Arr::pull() being null and not an array:

array_key_exists() expects parameter 2 to be array, null given

Line 27 already defines a default value for the client part of the configuration so it's not actually required to have any configuration to create the instance of RedisManager.

After this change having no Redis configuation at all will still result in an InvalidArgumentException if the Redis connection is used:

Redis connection [default] not configured.

But this is more descriptive and easier to debug than the array_key_exists error.