Exporting fails when no custom resources are present in project

```
Couldn't open directory at path "res://.godot/exported".
res://addons/mod_exporter/ModZipExporter-master/plugin.gd:117 - Out of bounds get index '0' (on base: 'PackedStringArray')
```
https://github.com/Ryhon0/ModZipExporter/blob/master/plugin.gd#L117