Make executor_class optional in trace class

Otherwise `Schema.new_trace` will require `executor_class` as a keyword
arg which is the wrong place to pass it in.

lib/graphql/batch/setup_multiplex.rb

@@ -14,7 +14,7 @@ def after_multiplex(multiplex)
     end
 
     module Trace
-      def initialize(executor_class:, **_rest)
+      def initialize(executor_class: nil, **_rest)
         @executor_class = executor_class
         super
       end