Make the exporter work when nothing is selected

When you first open a brand new file in Blender, there are no active objects in the scene. If you try to export, the Three.js option is grayed out. The hack to get around this is to select an object. Even if you de-select it, the `bpy.context.scene.objects.active` (because Blender) - and now the exporter can work.

The end result of this fix should be to be able to remove this hack, and still have the exporter work:

https://github.com/AndrewRayCode/toasted-potato/blob/master/test/scripts/export-from-blender.py#L15-L17