Fix StopIteration when computing mass of a compound containing an empty compound (#2078)

[youdie006]

Fixes #2078.

Repro

from cadquery.occ_impl.shapes import compound

compound().Volume()            # -> 0.0  (fine)
compound(compound()).Volume()  # -> raises StopIteration

A bare empty compound reports a mass of 0.0, but a non-empty compound whose first child is an empty compound raises StopIteration instead.

Root cause

Shape._mass_calc_function picks the mass function from the first non-compound descendant of a compound. It guarded the top-level empty case with if obj: but then descended with a bare next(iter(child)):

while child.ShapeType() == "Compound":
    child = next(iter(child))   # empty child compound -> bare StopIteration

Shape.__iter__ yields nothing for an empty compound, so next() raises. This is self-inconsistent: a bare empty compound already resolves to 0.0, so nesting it inside another compound must not flip that into a crash.

Fix

Use the two-argument next(..., None) sentinel so both the top-level-empty case (unchanged) and the nested-empty case resolve to SOLID -> 0.0. Non-empty compounds still descend to their first non-compound child exactly as before.

The same helper backs Volume(), Area(), and centerOfMass(), so all three are fixed (verified: nested-empty centerOfMass() now returns (0, 0, 0)).

Test

Adds test_nested_empty_compound_mass to tests/test_shapes.py, asserting the bare-empty control plus Volume()/Area() of a nested-empty compound are 0.0. Fails with StopIteration before the fix, passes after.

Thanks to @johnbeard for the clear report and minimal repro.

---

AI-assisted: this fix was prepared with the help of an AI coding assistant and reviewed by me.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.71%. Comparing base (d6729f5) to head (e426890).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2079      +/-   ##
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- Coverage   95.72%   95.71%   -0.01%     
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  Files          30       30              
  Lines        9533     9531       -2     
  Branches     1421     1420       -1     
==========================================
- Hits         9125     9123       -2     
  Misses        253      253              
  Partials      155      155              

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