Build "empty" nginx config when there are no web processes

[josegonzalez]

Description of feature

Currently, if a user creates an app and any of the following are true:

• either doesn't deploy it
• there are no web processes specified in the Procfile
• there are no running web processes (after a docker restart or similar)

What we should do is create a dummy nginx.conf that null-routes and only shows an error page for users. This way there isn't any ambiguity as to whether the app is created property or not.

Source for this is a conversation in slack where a user created an app and added letsencrypt, but then it wasn't resolving because we don't create a dummy nginx config.