Fix for hardcoded fill value for local grid map in trace worker

[BrianMichell]

Resolves #661

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.69%. Comparing base (c3ba558) to head (c4b061e).
⚠️ Report is 160 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mdio/segy/_workers.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #666      +/-   ##
==========================================
+ Coverage   85.30%   86.69%   +1.38%     
==========================================
  Files          46       80      +34     
  Lines        2219     4321    +2102     
  Branches      306      284      -22     
==========================================
+ Hits         1893     3746    +1853     
- Misses        281      500     +219     
- Partials       45       75      +30     

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[tasansal]

LGTM, but what is puzzling to me is, how did the patch coverage go to 50% ?