fix: ensure hostname defaults to nil

This prevents Swagger from putting a URL in the generated documentation.

Asana: https://app.asana.com/0/584764604969369/1205684882303815

apps/api_web/config/prod.exs

@@ -18,7 +18,7 @@ config :api_web, ApiWeb.Endpoint,
       shutdown_timeout: 90_000
     ]
   ],
-  url: [scheme: "https", port: 443],
+  url: [scheme: "https", host: nil, port: 443],
   server: true,
   cache_static_manifest: "priv/static/cache_manifest.json"