runtime error: nil realpath_root

When the `realpath_root` does not exist on the filesystem, I am getting a HTTP 500 (instead of a 404).  Here is the relevant information from the nginx log.

```
[error] 30#30: *6 lua entry thread aborted: runtime error: /etc/nginx/lua/htaccess.lua:95: attempt to concatenate field 'realpath_root' (a nil value)
```