Removing the "Null" node from assign_track_ids.

[yfukai]

This is a follow-up PR for #150, removing the "0" node from the track graph in assign_track_ids and updating relevant pieces of the code.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.03%. Comparing base (58cdd0d) to head (5fd9f0b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #153      +/-   ##
==========================================
- Coverage   88.03%   88.03%   -0.01%     
==========================================
  Files          49       49              
  Lines        3243     3242       -1     
  Branches      544      544              
==========================================
- Hits         2855     2854       -1     
  Misses        231      231              
  Partials      157      157              

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[JoOkuma]

Thanks, @yfukai.
LGTM, feel free to merge it.

[yfukai]

Hi @JoOkuma, thanks for the review! It seems that I don't have the right to merge, but I've also double-checked this, and I believe it is good to go!