Fix output record paths to avoid tombstone reuse

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

This pull request makes a minor update to the Add method in VertexPoolList.cs to ensure the Path property of outputRecord is properly initialized before it is cleared. This helps prevent potential null reference issues.

• Ensures outputRecord.Path is set to an empty list if it has the special Tombstone value before calling Clear() on it.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73%. Comparing base (7ef6c62) to head (dba6a00).

Files with missing lines	Patch %	Lines
src/PolygonClipper/VertexPoolList.cs	0%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@         Coverage Diff         @@
##           main    #38   +/-   ##
===================================
- Coverage    73%    73%   -1%     
===================================
  Files        30     30           
  Lines      3595   3596    +1     
  Branches    866    867    +1     
===================================
  Hits       2654   2654           
  Misses      718    718           
- Partials    223    224    +1     

Flag	Coverage Δ
unittests	73% <0%> (-1%)	⬇️

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