fix: throw error for apikey false in api event when not defined in api

[ShreyaGangishetty]

Issue #, if available:
#1328

Description of changes:
added a check if api_auth exists

Description of how you validated changes:
added an error case
make pr

Checklist:

• Write/update tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected
• Add/update example to examples/2016-10-31

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

[codecov-io]

Codecov Report

Merging #1365 into develop will increase coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #1365      +/-   ##
===========================================
+ Coverage    94.42%   94.46%   +0.04%     
===========================================
  Files           78       78              
  Lines         4554     4554              
  Branches       912      912              
===========================================
+ Hits          4300     4302       +2     
+ Misses         119      118       -1     
+ Partials       135      134       -1

Impacted Files	Coverage Δ
samtranslator/model/eventsources/push.py	90.9% <100%> (+0.37%)	⬆️

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