fix(spanner/spansql): fix SQL for CREATE CHANGE STREAM TableName; case

[nktks]

This PR fixes CreateChangeStream's SQL() outputs for the case parsed CREATE CHANGE STREAM TableName;.

Reproduce Code:
main.go

package main

import (
	"fmt"

	"cloud.google.com/go/spanner/spansql"
)

func main() {
	fmt.Printf(spansql.CreateChangeStream{
		Name: "csname",
	}.SQL())
}

expected

CREATE CHANGE STREAM csname

actual

go run main.go
CREATE CHANGE STREAM csname FOR