Support empty operation name

[ilyasgaraev]

Issue: an empty operation name causes the error in the redis-client library (dependency of Redis gem). Only relevant to Redis v5.

Reason:
Let's check the path from the graphql-anycable to redis-client:

1. GraphQL::Subscriptions::AnyCableSubscriptions#write_subscription (the fixed method) builds data hash with the operation_name key;
2. operation_name can be nil, because it's not a required attribute, and a subscription without a name is still valid;
3. graphql-anycable passes the data hash via the mapped_hmset method;
4. mapped_hmset sends HMSET command via redis_client;
5. redis_client checks the attributes here and triggers the error because one of the attributes (operation_name) is nil.

Here you can find a reason why redis-client doesn't support nil.

The issue is reproducible with Redis v5, you can remove the fix and the following error will be triggered in spec:

GraphQL::AnyCable with empty operation name subscribes channel to stream updates from GraphQL subscription
     Failure/Error: pipeline.mapped_hmset(SUBSCRIPTION_PREFIX + subscription_id, data)
     
     TypeError:
       Unsupported command argument type: NilClass

[Envek]

Thanks a lot for the pull request and such a detailed explanation, appreciate it.

Released in 1.1.6, please enjoy!