Fix SVG pack_untracked_polytomies bug

[hyanwong]

Fixes #3049

PR Checklist:

• Tests that fully cover new/changed functionality.
• Changelogs, if there are API changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.07%. Comparing base (73ef4cc) to head (98d89d7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3050      +/-   ##
==========================================
- Coverage   89.85%   87.07%   -2.78%     
==========================================
  Files          29       11      -18     
  Lines       32128    24666    -7462     
  Branches     5763     4556    -1207     
==========================================
- Hits        28868    21478    -7390     
+ Misses       1859     1824      -35     
+ Partials     1401     1364      -37     

Flag	Coverage Δ
c-tests	86.69% <ø> (ø)
lwt-tests	80.78% <ø> (ø)
python-c-tests	89.05% <ø> (ø)
python-tests	?

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see 18 files with indirect coverage changes