Output "empty" groups to preserve hierarchy.

Hello,

A while ago EMPTY groups were commented out.  This however breaks some valid hierarchies.  For instance see [this model] (https://github.com/yuriknsk/tu154b/blob/master/Model/exterior.ac) (press View Raw to download - ~5MB): when you import it into Blender for the first time there's a certain short top-level hierarchy, but after the export and import back all "empty" nodes (having children and nothing else) are "flattened".

The patch seem to work but I don't grok Blender or AC3D so I can't tell for sure if the change is complete and correct.

Thanks in advance for considering the patch!