Empty leaf node in assembly STEP export

[lorenzncode]

This is a follow up to #993.

The following results in a STEP with empty leaf node "toplevel" due to the empty top/root object.

import cadquery as cq

box = cq.Workplane().box(1, 2, 3)

assy = cq.Assembly(name="toplevel")
assy.add(box, name="box1")
assy.add(box, name="box2", loc=cq.Location((4, 0, 0)))

assy.save("boxes.step")

I think this leaf node can be considered extraneous. Is there a use case for the empty leaf node? If not, I'd suggest to remove it.

Compare with the toVTK based export formats where there is a conditional check on shapes:

cadquery/cadquery/occ_impl/assembly.py

Line 192 in d5806e3

if assy.shapes:

[adam-urbanczyk]

AFAIR it represents a Location - nodes always have a transformation but not always shapes.