Return empty shapes instead of throwing exceptions if nacelle or pylon definition is missing

[joergbrech]

Description

Fixes #779.

TiGL previously threw a CTiglError when trying to display an aircraft configuration containing an engine without a nacelle definition (e.g., an engine buried in the fuselage) or a pylon without geometry segments. This prevented the aircraft from being displayed at all in TiGLCreator.

Changes

• src/engine_nacelle/CCPACSEnginePosition.cpp: BuildLoft() now returns an empty PNamedShape() instead of throwing when the engine has no nacelle definition. This matches the established pattern used by other components like CCPACSAircraftModel::BuildLoft() and CCPACSDuctAssembly::BuildShape().
• src/engine_pylon/CTiglEnginePylonBuilder.cpp: BuildShape() now returns an empty PNamedShape() instead of throwing when the pylon has no segments.
• src/geometry/CTiglAbstractGeometricComponent.cpp: Added null checks in CalcBoundingBox(), GetMirroredLoft(), and GetIsOn() to handle the case where the loft is null, preventing crashes in dependent methods.

How Has This Been Tested?

• Manually tested opening files in TiGLCreator.
• A new unit test has been added. The existing tests run without failure.

Screenshots, that help to understand the changes(if applicable):

Checklist:

Task	Finished	Reviewer Approved
At least one test for the new functionality was added.	yes does not apply	OK
New classes have been added to the Python interface.	yes does not apply	OK
The code is properly documented with doxygen docstrings	yes does not apply	OK
Changes are documented at the top of ChangeLog.md	yes does not apply	OK

[codecov]

Codecov Report

❌ Patch coverage is 65.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.03%. Comparing base (8350f01) to head (1db04c9).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/engine_nacelle/CCPACSEnginePosition.cpp	42.85%	4 Missing ⚠️
src/geometry/CTiglAbstractGeometricComponent.cpp	75.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1323      +/-   ##
==========================================
+ Coverage   72.96%   73.03%   +0.06%     
==========================================
  Files         322      322              
  Lines       28290    28297       +7     
==========================================
+ Hits        20643    20667      +24     
+ Misses       7647     7630      -17     

Flag	Coverage Δ
unittests	73.03% <65.00%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/engine_pylon/CTiglEnginePylonBuilder.cpp	22.22% <100.00%> (+22.22%)	⬆️
src/geometry/CTiglAbstractGeometricComponent.cpp	79.16% <75.00%> (-1.79%)	⬇️
src/engine_nacelle/CCPACSEnginePosition.cpp	30.76% <42.85%> (+16.48%)	⬆️

... and 3 files with indirect coverage changes

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[svengoldberg]

Looks good to me!

I just have a small question...
After that, for me, the PR is ready to merge

[svengoldberg]

Just to be safe, I am not too familiar with the lofting of nacelles...

Is it impossible that this code part might throw an error, because of the if-check before?
In the old code, there is this try/catch block