Omit CodePipelineEventDetailType when it's missing in the payload

[calavera]

The examples in testdata don't include this field, so I'm guessing it's optional.

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[codecov-commenter]

Codecov Report

Merging #427 (c11063f) into main (507ad66) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #427   +/-   ##
=======================================
  Coverage   71.66%   71.66%           
=======================================
  Files          19       19           
  Lines        1041     1041           
=======================================
  Hits          746      746           
  Misses        228      228           
  Partials       67       67           

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