struct tag 'optional' is not work for query args

[fry95116]

it seems that the struct tag optional is not working

go version: 1.17
thunder version: github.com/samsarahq/thunder v0.5.0
minial demo:

package main

import (
	"net/http"

	"github.com/samsarahq/thunder/graphql"
	"github.com/samsarahq/thunder/graphql/introspection"
	"github.com/samsarahq/thunder/graphql/schemabuilder"
)

func main() {
	schema := schemabuilder.NewSchema()

	type Inner struct {
		OptionalValue string
		RequiredValue string
	}

	query := schema.Query()
	query.FieldFunc("inner", func(input struct {
		OptionalInput string `graphql:",optional"`
		RequiredInput string
	}) Inner {
		return Inner{
			OptionalValue: input.OptionalInput,
			RequiredValue: input.RequiredInput,
		}
	})

	builtSchema := schema.MustBuild()
	introspection.AddIntrospectionToSchema(builtSchema)

	http.Handle("/graphql", graphql.HTTPHandler(builtSchema))

	if err := http.ListenAndServe(":3030", nil); err != nil {
		panic(err)
	}

}

and in Graphql playground, it showed that arg OptionalInput is required