Exception when reading user property of request when authenticated with a service principal.

I have an example project which illustrates the issue.

https://github.com/shaneholder/managed-identity-oauth
clone the repo
```
cd terraform
terraform apply --auto-approve
cd ../api
npm install
npm run deploy
cd ../client
npm install
node ./call.js
```

you'll see that the response has an error property populated with `{ isAzureFunctionsSystemError: true }`

in the console logs of the azure function app you will see.

```
Error reading user info: AzFuncSystemError: Internal error: Expected value to be neither null nor undefined: user-name
at nonNullValue (/home/site/wwwroot/node_modules/@azure/functions/dist/azure-functions.js:2791:15)
at extractHttpUserFromHeaders (/home/site/wwwroot/node_modules/@azure/functions/dist/azure-functions.js:2078:54)
at get user (/home/site/wwwroot/node_modules/@azure/functions/dist/azure-functions.js:1852:127)
at handler (file:///home/site/wwwroot/src/functions/user.js:31:24)
at InvocationModel. (/home/site/wwwroot/node_modules/@azure/functions/dist/azure-functions.js:215:46)
at Generator.next ()
at /home/site/wwwroot/node_modules/@azure/functions/dist/azure-functions.js:121:71
at new Promise ()
at __webpack_modules__../src/InvocationModel.ts.__awaiter (/home/site/wwwroot/node_modules/@azure/functions/dist/azure-functions.js:117:12)
at InvocationModel.invokeFunction (/home/site/wwwroot/node_modules/@azure/functions/dist/azure-functions.js:213:16) {
isAzureFunctionsSystemError: true
}
```
I believe this happens because the header `x-ms-client-principal-name` is not populated by Easy Auth when an API is called using a service principal and the code in extractHttpUserFromHeaders.ts is using the nonNullValue() function to verify the value is not null.