fix: raise InvalidEventException for incorrect Properties field usage for Api event type

[mndeveci]

Issue #, if available:
N/A

Description of changes:
Fix validation for Properties field of HttpApi event source for Serverless Function.

Description of how you validated changes:
Tested with the case where Properties field would be an empty string, and added error test cases to cover that.

Checklist:

• Add/update unit tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• Add/update integration tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected
• Do these changes include any template validations?
  • Did the newly validated properties support intrinsics prior to adding the validations? (If unsure, please review Intrinsic Functions before proceeding).
    • Does the pull request ensure that intrinsics remain functional with the new validations?

Examples?

Please reach out in the comments, if you want to add an example. Examples will be
added to sam init through https://github.com/awslabs/aws-sam-cli-app-templates/

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.39%. Comparing base (e7a1496) to head (f0bc4e1).
Report is 1188 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2393      +/-   ##
===========================================
+ Coverage    93.58%   94.39%   +0.81%     
===========================================
  Files           90       98       +8     
  Lines         6124     7211    +1087     
  Branches      1260     1490     +230     
===========================================
+ Hits          5731     6807    +1076     
- Misses         183      196      +13     
+ Partials       210      208       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.