Ignore subscription publish with empty subscribed_fields

As written, `Absinthe.Subscribe.publish()` will still broadcast via
pubsub every single mutation results, even if the subscribed fields is
empty. This is because get_subscription_info() will return an empty
array if it does now find any subscribed fields. This is passed down all
the way to the pubsub.

Absinthe.Subscription.Local ultimately ignores this when attempting to
iterate over an empty list.

This can result in significant network traffic among nodes that
ultimately does nothing. When deployed on the cloud in a multi-az
deployment, this can result in significant increase in network traffic
charges.

This patch adds a clause where empty subscribed_fields is ignored.

lib/absinthe/subscription.ex

@@ -74,6 +74,8 @@ defmodule Absinthe.Subscription do
           term,
           Absinthe.Resolution.t() | [subscription_field_spec]
         ) :: :ok
+  def publish(_pubsub, _mutation_result, []), do: :ok
+
   def publish(pubsub, mutation_result, %Absinthe.Resolution{} = info) do
     subscribed_fields = get_subscription_fields(info)
     publish(pubsub, mutation_result, subscribed_fields)