FIX: Add error message if extension is started with an empty HFSS 3D Layout design

[ecoussoux-ansys]

This PR provides a fix for #6780 by introducing an error message in case the cutout extension is started on an empty HFSS 3D Layout design.

This case is not supported by the __load_objects_net() method (which is part of the __init__() of the extension) responsible for loading the objects by net from the EDB modeler: Therefore an error is now raised to avoid subsequent crash of the code in this situation.
A corresponding unit test is added.

Close #6780.

[eblanco-ansys]

LGTM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.32%. Comparing base (712cc33) to head (347207d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6822   +/-   ##
=======================================
  Coverage   83.32%   83.32%           
=======================================
  Files         246      246           
  Lines       77442    77445    +3     
=======================================
+ Hits        64527    64531    +4     
+ Misses      12915    12914    -1     

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