Use valid_isolated_input in field_ast

lib/graphql/relay/walker/query_builder.rb

@@ -106,7 +106,11 @@ module GraphQL::Relay::Walker
 
       # Bail unless we have the required arguments.
       return unless field.arguments.reject do |_, arg|
-        valid_input?(arg.type, nil)
+        if GraphQL::VERSION >= "1.5.6"
+          type.valid_isolated_input?(arg.type)
+        else
+          valid_input?(arg.type)
+        end
       end.all? do |name, _|
         arguments.key?(name)
       end