Support NO_UPDATE in ActionCable subscriptions

[jqr]

Looks like this may be the fix for #3189 and similar empty subscription updates I'm seeing in my application.

Applies the same pattern to ActionCableSubscriptions as Subscriptions, which just checks if execute_update returns nil and skips the delivery. See

graphql-ruby/lib/graphql/subscriptions.rb

Lines 141 to 146 in bace1e4

	def execute(subscription_id, event, object)
	res = execute_update(subscription_id, event, object)
	if !res.nil?
	deliver(subscription_id, res)
	end
	end

Given this is a subclass, I'm guessing there's a better way to rely on the superclass behavior and ensure all subclasses inherit this important feature. Open to suggestions.

[rmosolgo]

Sorry for my slow response on this. Looks good to me! I think putting this check in the subclass is fine. In practice, different subscription implementations have to manage their own storage and transport, so this check would go somewhere different in each one.