Pythonic replacement for the mutable default for vertices argument of Volume class constructor

[oskooi]

The vertices argument of the Volume class constructor uses a mutable default (a list). While this is not a functional bug since the list object is never mutated as vertices is a throwaway parameter used only to compute center and size, it is a code smell as it would fail a linter like pylint or ruff with B006/W0102. This PR replaces the mutable default with an idiomatic Python convention – None as a sentinel for "no value provided" is universally understood.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.90%. Comparing base (f29a8c7) to head (3facd51).
⚠️ Report is 108 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3159      +/-   ##
==========================================
+ Coverage   73.81%   73.90%   +0.09%     
==========================================
  Files          18       18              
  Lines        5423     5454      +31     
==========================================
+ Hits         4003     4031      +28     
- Misses       1420     1423       +3     

Files with missing lines	Coverage Δ
python/simulation.py	77.56% <100.00%> (+0.15%)	⬆️

... and 1 file with indirect coverage changes

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