Raise AuthenticationCredentialsNotFoundException if Principal is not present and declared as not optional

[koenpunt]

The issue with a non-null principal argument is more visible in Kotlin then it is in Java; in Java even when a function argument is annotated as non-null, it still accepts a null value for that parameter, but in Kotlin this results in a "can not assign null to non-null parameter" error.

By throwing a security error when the argument is non-null, and the principal isn't present in the security context, authentication errors can be handler in an exception resolver class, instead of resulting in an arbitrary internal error.

Fixes #714

[rstoyanchev]

This looks good so far. However, we also have AuthenticationPrincipalArgumentResolver that probably needs a similar update.

[koenpunt]

Updated it to reuse the doResolve method. Not sure if that's alright, or that maybe that method should now be moved to a more common place?

[rstoyanchev]

This is now processed, thanks for the changes! I've made some small adjustments, notably to treat Mono as optional since can it be empty.

[koenpunt]

Great! Although I didn't expect this to get merged right away, and I think it might need some additional tests for the @AuthenticationPrincipal?

[rstoyanchev]

You can submit a PR in draft mode in that case. Overall seems alright to me. The resolver for @AuthenticationPrincipal delegates to the resolver for Principal and that is already tested. If you'd like to add more tests, send another PR.