Non-fronted connections to the broker no longer work since 9fdfb3d1

The changes in !182 (merged) caused a regression for client configurations that only provide a url= option and not a front= option. The check at https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/blob/4fa43a88927e42fb2e3864f706d3e3c72fd0dca3/client/lib/rendezvous_http.go#L49 is never false, causing a connection to a blank URL to happen if no front is provided:

2023/10/26 14:46:44 Negotiating via HTTP rendezvous...
2023/10/26 14:46:44 Target URL:  snowflake-broker.torproject.net
2023/10/26 14:46:44 Front URL:   
2023/10/26 14:46:44 WebRTC: closing DataChannel
2023/10/26 14:46:44 WebRTC: closing PeerConnection
2023/10/26 14:46:44 WebRTC: Closing
2023/10/26 14:46:44 WebRTC: dial tcp :443: connect: connection refused  Retrying...