FBX IO: Fix Empty export type control also affecting armatures
The exported type of the NodeAttribute for Empty Objects can be
controlled using a custom property called 'fbx_type' on the Object. This
feature was also being applied to Armature Objects because they use the
same function for exporting their NodeAttribute.
A check has now been added such that the exported type can only be
changed by the 'fbx_type' custom property when the Object is an Empty.
Pull Request: https://projects.blender.org/blender/blender-addons/pulls/104887