Export: Ignore empty vertex groups

[Brachi]

If there's a single empty vertex group, e.g., a vertex group with no weights assigned the export will fail with an obscure traceback.

Error: Python: Traceback (most recent call last):
  File "/blender/3.6/scripts/addons/albam/blender_ui/export_panel.py", line 224, in execute
    self._execute(item)
  File "/blender/3.6/scripts/addons/albam/blender_ui/export_panel.py", line 234, in _execute
    vfiles = export_function(item.bl_object)
  File "/blender/3.6/scripts/addons/albam/engines/mtfw/mesh.py", line 733, in export_mod
    _serialize_meshes_data(bl_obj, bl_meshes, src_mod, dst_mod, materials_map, bone_palettes))
  File "/blender/3.6/scripts/addons/albam/engines/mtfw/mesh.py", line 1164, in _serialize_meshes_data
    mesh_weight_bounds = _calculate_weight_bounds(
  File "/blender/3.6/scripts/addons/albam/engines/mtfw/mesh.py", line 1486, in _calculate_weight_bounds
    weight_bound = _calculate_vertex_group_weight_bound(
  File "/blender/3.6/scripts/addons/albam/engines/mtfw/mesh.py", line 1602, in _calculate_vertex_group_weight_bound
    min_x = min((v[0] for v in vertices_in_group_bone_space))
ValueError: min() arg is an empty sequence

It won´t tell you which mesh, and it will keep failing until all are fixed.

The albam_reloaded tool "Remove empty vertex groups" is almost mandatory then, providing a terrible user experience for first-time users . Empty weight groups can be safely ignored.