Allow empty (blank) scene files

This is just a minor annoyance I discovered while trying to work with bevy scenes.

When prototyping some code that loads bevy scenes from asset files (the `.scn.ron` kind), you might initially not have any content in a particular scene file, while you are just trying to stub things out.

Simply creating a blank file as a placeholder does not work, however.

```
bevy_asset::asset_server: encountered an error while loading an asset: 2:1: Expected opening `[`
```

This is because Bevy expects that the scene file must contain a RON array.

To workaround this, I have to open my placeholder/stub scene asset file in a text editor, to add an empty array into it:

```
[]
```

and then it loads successfully.

This is quite annoying.

It would be very nice if Bevy could allow loading completely blank scene files without erroring.