Allow starting http-proxy without DISCORD_TOKEN when Authorization header is always provided

[Sayrix]

I'm running http-proxy as a gateway for multiple bots where every incoming request already includes an Authorization header. Right now the proxy fails to start if DISCORD_TOKEN is not set. Providing a dummy DISCORD_TOKEN works as a workaround, but it would be nicer if the proxy could start without a default token when users plan to always use the request's Authorization header.