fix(event_handler): fix ALB resolver returns when response body is None

[leandrodamascena]

Issue number: closes #8186

Summary

ALB resolver converts body=None to body="" to prevent ALB 5xx errors, but this conversion happens before the response validation middleware runs. Validation then sees "" instead of None and fails with none_required for handlers returning -> None.

Fix: in the validation middleware, treat empty string body as None when the return type is None.

Changes

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User experience

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.72%. Comparing base (5b8ba36) to head (d52eab7).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8194   +/-   ##
========================================
  Coverage    96.72%   96.72%           
========================================
  Files          286      286           
  Lines        14341    14342    +1     
  Branches      1200     1200           
========================================
+ Hits         13871    13872    +1     
  Misses         341      341           
  Partials       129      129           

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud