test: improve isnan function SQL test coverage

[2010YOUY01]

Which issue does this PR close?

• Closes #.

Rationale for this change

Codecov reports low test coverage for this function, so this PR adds more SLT tests for it.

The tests were AI-generated, and I manually verified that the results are correct.

See Codecov bot comment -> ☔ View full report in Codecov by Harness -> Indirect Changes for the updated test coverage.

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.72%. Comparing base (02a902c) to head (1a72cbc).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23754      +/-   ##
==========================================
+ Coverage   80.71%   80.72%   +0.01%     
==========================================
  Files        1089     1089              
  Lines      368293   368750     +457     
  Branches   368293   368750     +457     
==========================================
+ Hits       297250   297682     +432     
+ Misses      53332    53323       -9     
- Partials    17711    17745      +34     

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[alamb]

Thanks @2010YOUY01 and @nuno-faria -- this is great