Fix segfault when loading an assembly with no children from xml/xbf

[HaozheZhang6]

Loading an assembly that holds a single shape and no children crashes the interpreter:

import cadquery as cq

assy = cq.Assembly(cq.Solid.makeBox(1, 1, 1))
assy.export("a.xml")
cq.Assembly.load("a.xml")     # Fatal Python error: Segmentation fault

Same for .xbf. Adding a single child makes it go away, which is why the existing round-trip tests do not hit it -- every fixture has children. STEP is unaffected.

Cause: such an assembly has no TDataStd_Name on its top-level label after the XCAF round trip, and _importDoc calls

name_attr = TDataStd_Name()
top_level_label.FindAttribute(TDataStd_Name.GetID_s(), name_attr)
assy.name = str(name_attr.Get().ToExtString())

FindAttribute segfaults on a label that does not carry the attribute rather than returning false:

Fatal Python error: Segmentation fault
  File "cadquery/occ_impl/importers/assembly.py", line 378 in _importDoc
  File "cadquery/occ_impl/importers/assembly.py", line 227 in importXml

The module already has a helper that gets this right -- _get_name checks IsAttribute before calling FindAttribute -- so use it, and keep the generated name when the file does not carry one.

Test: test_assembly_without_children_roundtrip parametrized over step/xml/xbf. On master the xml and xbf cases take the whole pytest process down with SIGSEGV.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.79%. Comparing base (6d741ee) to head (aa9b454).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2065   +/-   ##
=======================================
  Coverage   95.79%   95.79%           
=======================================
  Files          30       30           
  Lines        9435     9435           
  Branches     1405     1406    +1     
=======================================
  Hits         9038     9038           
  Misses        242      242           
  Partials      155      155           

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