Avoid additional HashMap allocation for Cumulative aggregation

[utpilla]

Follow-up to #2343

Changes

• Avoid the additional HashMap allocation for Cumulative aggregation by initializing the HashMap lazily

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.5%. Comparing base (1cecaea) to head (3e81ff1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2352     +/-   ##
=======================================
- Coverage   79.5%   79.5%   -0.1%     
=======================================
  Files        123     123             
  Lines      21379   21381      +2     
=======================================
  Hits       17003   17003             
- Misses      4376    4378      +2     

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[lalitb]

nice.

[cijothomas]

Nice!!