Empty NEXT_PUBLIC_SITE_URL env var breaks checkout in local testing

[chriscarrollsmith]

The example .env.local file contains this text:

# Update this with your stable site URL only for the production environment.
NEXT_PUBLIC_SITE_URL=

If the user leaves this variable empty rather than deleting it, the template fails to create a checkout session during local testing. That's because getURL in 'helpers.ts' is only using the localhost fallback when NEXT_PUBLIC_SITE_URL is non-existent, not when it's empty.

[leerob]

#278