Handle when HttpApi resource doesn't have properties

[aahung]

Issue #, if available:

Description of changes:

Description of how you validated changes:

Checklist:

• Add/update tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected

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

Merging #2221 (d679632) into develop (e7a1496) will increase coverage by 0.96%.
The diff coverage is 98.42%.

@@             Coverage Diff             @@
##           develop    #2221      +/-   ##
===========================================
+ Coverage    93.58%   94.54%   +0.96%     
===========================================
  Files           90       95       +5     
  Lines         6124     6566     +442     
  Branches      1260     1322      +62     
===========================================
+ Hits          5731     6208     +477     
+ Misses         183      166      -17     
+ Partials       210      192      -18     

Impacted Files	Coverage Δ
samtranslator/model/lambda_.py	93.10% <ø> (ø)
samtranslator/plugins/globals/globals.py	99.05% <ø> (ø)
samtranslator/translator/logical_id_generator.py	100.00% <ø> (+9.09%)	⬆️
samtranslator/region_configuration.py	77.77% <63.63%> (-22.23%)	⬇️
samtranslator/swagger/swagger.py	94.02% <94.23%> (+0.66%)	⬆️
samtranslator/model/api/api_generator.py	94.69% <96.55%> (+0.33%)	⬆️
samtranslator/__init__.py	100.00% <100.00%> (ø)
samtranslator/feature_toggle/dialup.py	100.00% <100.00%> (ø)
samtranslator/feature_toggle/feature_toggle.py	100.00% <100.00%> (+12.16%)	⬆️
samtranslator/intrinsics/actions.py	98.75% <100.00%> (+0.03%)	⬆️
... and 36 more

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