GLTF2 exporter ignores childless nodes in the scene hierarchy

Bug

The GLTF2 Exporter seems to be ignoring any Mesh or TransformNode that does not have children in the scene hierarchy before exporting/serializing the scene.

Here is the line of code that seems to be responsible for this behaviour: https://github.com/BabylonJS/Babylon.js/blob/v4.0.0/serializers/src/glTF/2.0/glTFExporter.ts#L1342

Problem

This can be troublesome as sometimes it is useful to have an empty transform that stores a location (for example a spawn point for a prefab instance, a player, etc) or an orientation, etc. I’m not sure if this is something that is necessary because of the way the GLTF specs are, if it is I would be glad to know more about this subject.

Notes

The repro code in the Babylon Playground creates two sets of Mesh and TransformNode instances. If you export them and reimport them in the Babylon Sandbox for example, you will noticed in the Inspector that the nodes without children have been ignored at export-time.

These instructions can also be found in the repro playground:

NOTE: To export to GLB:
1- Select the GEAR icon in the Babylon Inspector toolbar above
2- Select Inspector in the displayed menu
3- In the Inspector panel that should have opened to the right, select the WRENCH icon.
4- In the SCENE EXPORT section, select Export to GLB
5- Load the downloaded file in https://sandbox.babylonjs.com/ by dragging it in the middle of the scene.
6- Open the Babylon Inspector at the bottom of the window
7- Expand the + Nodes section
8- Notice that only meshes and transform nodes that have children haven been exported

Versions

Tested in BabylonJS 4.0.3, seems to also be occurring in latest version (master branch).

Playground

• Bug repro on playground

Expected result

Exported scene should have all the meshes and transform nodes regardless of if they have children or not?

Current result

Any Mesh or TransformNode without children will be ignored when exporting to GLB

Workaround

It seems like an easy workaround would be to add a temporary child mesh to the meshes we wish to preserve.

[deltakosh]

Well this is more a bug so no worries for backward compat

[deltakosh]

@osmanzeki fancy doing a PR to fix it?