Fix nginx template: guard proxy pass for initial/clean deployments

[gideonmaina]

@VinneyJ Production deploy dispatch simulation works now for an app that was never deployed before. This is a separate nginx issue that will be addressed. However, since sensors prod has been deployed before and nginx configurations exist, the workflow will dispatch without any issues.

Originally posted by @gideonmaina in #165 (comment)

[gideonmaina]

The location /{ } part of both http and https blocks should have a conditional guard for zero listeners on first deploy @VinneyJ

    ##End CORS

    {{ if $.DOKKU_APP_LISTENERS }}
    proxy_pass  http://{{ $.APP }}-{{ $upstream_port }};
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
    proxy_set_header Host $http_host;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_set_header X-Forwarded-For $remote_addr;
    proxy_set_header X-Forwarded-Port $server_port;
    proxy_set_header X-Request-Start $msec;
    {{ else }}
    return 503;
    {{ end }}