Empty doc_root directive not ignored

According to http://php.net/manual/en/ini.core.php#ini.doc-root, the `doc_root` directive should only be used if non-empty. However, hhvm seems to use it if it is empty.

`php.ini`

```
doc_root =
```

`index.php`

```

```

When `hhvm -m server -p 8088 -d hhvm.server.source_root=./` is running, the command `curl localhost:8088` returns `RequestInitDocument Not Found`, and the hhvm command line shows `Notice: File could not be loaded: /index.php`. Replacing `php.ini` with a blank file fixes this, as does running `php -S localhost:8088 ./index.php` instead of hhvm.