[FBX] Do not rename null nodes. Preserve information in node metadata.

Changing node names is a problem because you cannot map them in different fbx files.
For example if you have a t-pose skeleton with a mesh in an fbx and a bunch on animation fbx without the mesh, in the animation fbx the skeletons will be renamed because the fbx doesn't have a mesh in it.

Instead i used the metadata with a "IsNull" entry to preserve this information.

Metadata is turning out to be a very useful way to keep information of fbx, that are important but have no meaning in assimp.
The problem is that is very basic right now and i would like to work on it some more. :)