Bone without assigned vertices breaks animation

[FaisalZ]

I recently used this blender exporter to export an armature animation to xml3D.
One of my bones did not have any vertices assigned, which detained the browser
to load the scene at all.
I know that it is no good practice to have a bone with no assigned vertices, but
it can happen and is certainly allowed within blender. At least it should not break the
entire scene.