Allow App Service callers without a principal name

[Hyun Lee (dlwhdgus0810)]

Fixes #374

Problem

App Service Authentication sends no x-ms-client-principal-name header when the caller has no name claim. A service principal that calls the API with a client credentials token is one such caller. The other headers are still sent: x-ms-client-principal-id, x-ms-client-principal-idp and x-ms-client-principal.

extractHttpUserFromHeaders required the name header through nonNullValue, so reading request.user threw:

AzFuncSystemError: Internal error: Expected value to be neither null nor undefined: user-name

Changes

• The AppService branch uses an empty username when x-ms-client-principal-name is missing. The id and identity provider headers are still required.
• The username doc comment in types/http.d.ts says when it is empty.

The type stays string, so existing code keeps compiling. username?: string would describe the service principal case more exactly, but it changes the public type. I can switch to that if you prefer it.

Testing

• New test in test/http/extractHttpUserFromHeaders.test.ts: AppService headers without x-ms-client-principal-name give an AppService user with an empty username. Without the change, the test fails with the error above.
• npm test: 463 passing.
• eslint, prettier --check and npm run build pass.

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[Hyun Lee (dlwhdgus0810)]

@microsoft-github-policy-service agree