fix(event-handler): body to empty string in CORS preflight (ALB non-compliant)

[heitorlessa]

Issue number: #1248

Summary

Changes

Please provide a summary of what's being changed

ALB doesn't follow HTTP CORS Spec and returns 502 when a body response is null. This PR returns an empty string to make it compliant.

User experience

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Checklist

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• Meet tenets criteria
• I have performed a self-review of my this change
• Changes are 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)

Acknowledgment

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[codecov-commenter]

Codecov Report

Merging #1249 (e545793) into develop (2302099) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1249   +/-   ##
========================================
  Coverage    99.88%   99.88%           
========================================
  Files          119      119           
  Lines         5423     5423           
  Branches       618      618           
========================================
  Hits          5417     5417           
  Misses           2        2           
  Partials         4        4           

Impacted Files	Coverage Δ
aws_lambda_powertools/event_handler/api_gateway.py	100.00% <100.00%> (ø)

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