Checks whether process.env.HOME exists

Checks whether process.env.HOME exists before calling Path.join
Was not able to start sinopia on my fresh windows install due to this error.
Wouldn't an easier way be below? You wouldn't even really need the `|| null` at the end since the try statement in the folder_exists function would catch the "emptyness"

```js
var xdg_config = process.env.XDG_CONFIG_HOME
    || process.env.HOME && Path.join(process.env.HOME, '.config')
    || null
```