empty_mesh initialization and callback issue for Chrono imports, Fixes #669

[TomMontagnon]

As specified in issue #669:

"this is a known issue in Blender + plugins that implement the frame
update callback. The solution is to save the Blender project and launch
the blender rendering via command line in headless mode (no GUI)."

Following this workflow requires closing Blender and launching the animation rendering in headless mode.

However, closing Blender resets the global variables, including empty_mesh. This means that when the callback runs during rendering, empty_mesh is None. Consequently, the following lines:

new_mesh = bpy.data.meshes.new('mesh_position_clones')
chobject.data = new_mesh

crash with a TypeError because chobject.data is not prepared to receive a mesh. This also causes only a subset of objects to be instantiated in the scene.

This PR ensures that empty_mesh is properly initialized before it is used in the callback, guaranteeing that all Chrono objects are correctly created and rendered in headless mode.