I am not sure why you would need an empty object with a rigid body, but here is my solution assuming you don’t want the rigid body to be visible .
A few things to consider: In Armory, a rigid body cannot directly be applied to an empty object, since rigid bodies need dimensions and empty’s don’t have. Also, Blender does not allow an empty to have a rigid body.
Now, as a solution, you can parent a mesh object to an empty and set the mesh to not render. But this mesh can still have rigid body. like so:
In this case, I have applied a cube type rigid body to the mesh object “Cube”.
Once this is done, you can now spawn the “Cube” object in Armory and set it invisible immediately each time. like so:
I have provided a blend file below that shows this example. I have enabled the debug console, so you could see where the spawned "Cube"s and their respective child “Empty” objects are located.
SpawnEmptyWithPhysics.blend (743.5 KB)
Hope this helps.