Fix panic when using flaps client without token

[joeriddles]

Currently the below code will panic, even though it does not appear that the /platform/regions endpoint requires authentication.

package main

import (
	"context"
	"fmt"
	"log"

	"github.com/superfly/fly-go/flaps"
)

func main() {
	ctx := context.Background()
	flap, err := flaps.NewWithOptions(ctx, flaps.NewClientOpts{})
	if err != nil {
		log.Fatal(err)
	}
	regions, err := flap.GetRegions(ctx, "")
	fmt.Print(regions)
}