Fix events map initialization to prevent race conditions

Initialization upon miss can lead to hard to debug scenarios where potentially a concurrent array will leak out but the value in the `@events` will be different.

as described here: ruby-concurrency/concurrent-ruby#970

lib/graphql/subscriptions/action_cable_subscriptions.rb

@@ -91,7 +91,13 @@ def initialize(serializer: Serialize, namespace: '', action_cable: ActionCable,
         # A per-process map of subscriptions to deliver.
         # This is provided by Rails, so let's use it
         @subscriptions = Concurrent::Map.new
-        @events = Concurrent::Map.new { |h, k| h[k] = Concurrent::Map.new { |h2, k2| h2[k2] = Concurrent::Array.new } }
+        @events = Concurrent::Map.new do |h, k|
+          h.compute_if_absent(k) do
+            Concurrent::Map.new do |h2, k2|
+              h2.compute_if_absent(k2) { Concurrent::Array.new }
+            end
+          end
+        end
         @action_cable = action_cable
         @action_cable_coder = action_cable_coder
         @serializer = serializer