fix(post-processing): handle missing values in cumulative operator

[villebro]

SUMMARY

If your data contains nulls, using the cumsum operator will cause gaps in the chart. This is because null values appear as nulls in the series instead of the cumulative value. To avoid this, we replace nulls with zeros before applying the operator, ensuring that all values are present in the series.

AFTER

Now gaps in the data won't cause the bar to be truncated:

BEFORE

Previously, missing values would cause the cumulative series to be missing in the chart, causing gaps in the graph:

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue: closes Cumsum Creating Discontinuous Lines #21093
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4c2e818) 69.16% compared to head (7f5039f) 69.16%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #26429      +/-   ##
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- Coverage   69.16%   69.16%   -0.01%     
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  Files        1948     1948              
  Lines       76042    76054      +12     
  Branches     8493     8493              
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+ Hits        52593    52601       +8     
- Misses      21269    21273       +4     
  Partials     2180     2180              

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hive	53.68% <0.00%> (-0.01%)	⬇️
mysql	78.07% <0.00%> (-0.01%)	⬇️
postgres	78.17% <0.00%> (-0.01%)	⬇️
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python	82.85% <100.00%> (-0.01%)	⬇️
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unit	55.86% <100.00%> (+0.06%)	⬆️

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[kgabryje]

LGTM

[JohnDietrich-Pepper]

Ah this fix really broke our charting. We wanted to show discontinuous lines when the data is null because the data is null and NOT zero particularly when the time period hasn't occurred yet. Now Superset is assuming all null data is zero which is not the case.
@villebro @rusackas