Fix incorrect IntPtr null check in FftUtils

[GrabYourPitchforks]

Caught by prototype CodeQL rule.

We should not compare an IntPtr value to null since it will always result as not equal. We should compare to IntPtr.Zero instead.

[Copilot]

Copilot reviewed 1 out of 1 changed files in this pull request and generated no comments.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.99%. Comparing base (87e1fec) to head (5c910dd).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7434      +/-   ##
==========================================
+ Coverage   68.98%   68.99%   +0.01%     
==========================================
  Files        1482     1482              
  Lines      273876   273876              
  Branches    28250    28254       +4     
==========================================
+ Hits       188944   188974      +30     
+ Misses      77550    77516      -34     
- Partials     7382     7386       +4     

Flag	Coverage Δ
Debug	68.99% <100.00%> (+0.01%)	⬆️
production	63.28% <100.00%> (+0.01%)	⬆️
test	89.46% <ø> (ø)

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Files with missing lines	Coverage Δ
src/Microsoft.ML.TimeSeries/FftUtils.cs	95.20% <100.00%> (ø)

... and 7 files with indirect coverage changes

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