Process non-empty environment variables in get_cgi_filename

Easier to debug when nginx sends empty variables.
Allows unsetting `SCRIPT_FILENAME` like in :
```
include /etc/nginx/fastcgi_params;
fastcgi_param   SCRIPT_FILENAME         "";
fastcgi_param   SCRIPT_NAME             $fastcgi_script_name;
fastcgi_param   DOCUMENT_ROOT           $document_root;
```