server.ts: remove invalid host fallback

If `environment.ui.host` is Falsy, then we attempt to fall back to
a host value of '/', which is invalid. I think that, if a user has
messed up their config so much that defaults in our configuration
interface don't work, then we should actually fail here.

server.ts

@@ -562,7 +562,7 @@ function createHttpsServer(keys) {
  */
 function run() {
   const port = environment.ui.port || 4000;
-  const host = environment.ui.host || '/';
+  const host = environment.ui.host;
 
   // Start up the Node server
   const server = app();