Compound containing empty Compound raises during Volume()

[johnbeard]

When computing the volume of a compound that itself contains an empty compound, a StopIteration exception is raised.

However the volume of the inner empty compound itself is computed to be 0.0 as expected.

To Reproduce

from cadquery.occ_impl.shapes import compound

empty_compound = compound()

empty_volume = empty_compound.Volume()  # returns 0.0
print("empty compound volume:", empty_volume)

bad = compound(empty_compound)  # top-level non-empty, first child empty
volume = bad.Volume()  # raises StopIteration

print("bad compound volume:", volume) # this line is never reached

Output:

empty compound volume: 0.0
Traceback (most recent call last):
  File "/tmp/cq _vol_test.py", line 9, in <module>
    volume = bad.Volume()  # raises StopIteration
  File "/home/john/src/klu/.venv/lib/python3.14/site-packages/cadquery/occ_impl/shapes.py", line 1118, in Volume
    return Shape.computeMass(self, tol)
           ~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/home/john/src/klu/.venv/lib/python3.14/site-packages/cadquery/occ_impl/shapes.py", line 824, in computeMass
    calc_function = Shape._mass_calc_function(obj)
  File "/home/john/src/klu/.venv/lib/python3.14/site-packages/cadquery/occ_impl/shapes.py", line 804, in _mass_calc_function
    child = next(iter(child))
StopIteration

Environment

OS: Arch Linux

Was CadQuery installed using Conda?: No, installed with pip

Version: 2.8.0, Python 3.14.6