Fix/more robust ptb parsing

[MartinKl]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 63.63636% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.67%. Comparing base (9ffe80a) to head (c7a9d91).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/importer/ptb.rs	63.63%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #501      +/-   ##
==========================================
- Coverage   84.69%   84.67%   -0.02%     
==========================================
  Files          69       69              
  Lines       24688    24697       +9     
  Branches    24688    24697       +9     
==========================================
+ Hits        20909    20913       +4     
- Misses       1473     1479       +6     
+ Partials     2306     2305       -1     

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