OptimalExtract: handle variance input with nans

[kecnry]

handle nans in the variance input similar to zeros: exclude those elements by modifying the mask

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.03%. Comparing base (7e7e4a6) to head (87eaad0).
⚠️ Report is 259 commits behind head on main.

Files with missing lines	Patch %	Lines
specreduce/extract.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
- Coverage   70.09%   70.03%   -0.07%     
==========================================
  Files           9        9              
  Lines         602      604       +2     
==========================================
+ Hits          422      423       +1     
- Misses        180      181       +1     

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