fix: support authorizer with no identity source specified

[frozenbonito]

Description

fix #1638

Motivation and Context

If the authorizer type is REQUEST and its caching is disabled, the identity source is not required.
See #1638 for details.

How Has This Been Tested?

I added no-identity-source-authorizer integration test.

[dnalborczyk]

thank you for the PR @frozenbonito !

when I publish your serverless.yml to aws npx serverless deploy and hit the route (with no headers) my response is:

{
  "message": null
}

whereas when I run it locally npx serverless offlin start I get:

{
  "message": "hello"
}

is that expected?

[frozenbonito]

@dnalborczyk
Sorry, it seems that package.json was missing.

[frozenbonito]

Fixed.

[cruzj6]

Will this be merged? Still having this issue