fix(event-handler): return dict on missing multi_value_headers

[heitorlessa]

Issue number: #3823

Summary

Addresses regression introduced in 2.33 when running Event Handler APIs locally when multiValueHeaders isn't present.

Changes

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User experience

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Checklist

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• Meet tenets criteria
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• PR title follows conventional commit semantics

Is this a breaking change?

RFC issue number:

Checklist:

• Migration process documented
• Implement warnings (if it can live side by side)

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues

Measures
0 Security Hotspots
No data about Coverage
2.1% Duplication on New Code

See analysis details on SonarCloud

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e14e768) 96.38% compared to head (c944e3b) 96.42%.
Report is 77 commits behind head on develop.

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3824      +/-   ##
===========================================
+ Coverage    96.38%   96.42%   +0.04%     
===========================================
  Files          214      214              
  Lines        10030    10093      +63     
  Branches      1846     1866      +20     
===========================================
+ Hits          9667     9732      +65     
+ Misses         259      258       -1     
+ Partials       104      103       -1     

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