Mark HTTP v2 JWT scopes as omitempty

[mattrjacobs]

Issue #, if available:
GH-317

Description of changes:
Mark HTTP v2 JWT scopes as omitempty

I observed this field as null in the wild, and others have as well:

• https://www.jeremydaly.com/verifying-self-signed-jwt-tokens-with-aws-http-apis/
• https://dev.to/martzcodes/token-authorizers-with-apigatewayv2-tricks-apigwv1-doesn-t-want-you-to-know-41jn

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #320 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #320   +/-   ##
=======================================
  Coverage   74.46%   74.46%           
=======================================
  Files          18       18           
  Lines         834      834           
=======================================
  Hits          621      621           
  Misses        150      150           
  Partials       63       63           

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[bmoffatt]

Thanks!