fix(alerts): Make sure null values aren't considered to be equal to 0

[kgopal492]

SUMMARY

Previously, the alert logic converted NULL results to 0.0 for comparison. However this incorrectly resulted in alerts triggered when the result is null if the alert is testing equality to 0. This logic uses np.nan to compare alerts with values.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@                    Coverage Diff                     @@
##             pinterest-release-4.1.1      #46   +/-   ##
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  Coverage                           ?   83.62%           
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  Files                              ?      535           
  Lines                              ?    38538           
  Branches                           ?        0           
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  Hits                               ?    32226           
  Misses                             ?     6312           
  Partials                           ?        0           

Flag	Coverage Δ
hive	48.92% <0.00%> (?)
postgres	76.86% <100.00%> (?)
presto	53.43% <0.00%> (?)
python	83.62% <100.00%> (?)
sqlite	76.33% <100.00%> (?)
unit	60.25% <0.00%> (?)

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