OAuth2AccessTokenInterceptor can handle Authentication Principal where principalName is null

[agileknight]

Is your feature request related to a problem? Please describe.
When OAuth2AccessTokenInterceptor encounters an Authentication Principal where getName is null, it passes the null value on to the OAuth2AuthorizedClientManager, which in practice usually results in a runtime exception like principalName cannot be empty.

Describe the solution you'd like
The code already checks for a null principal an in that case passes ANONYMOUS_AUTHENTICATION. It appears to also make sense to pass ANONYMOUS_AUTHENTICATION for cases with a non-null principal but a null principal name.

Describe alternatives you've considered
Handling a null principal name downstream or upstream appears more complicated and it feels more natural to follow the current implementation of employing the ANONYMOUS_AUTHENTICATION placeholder for cases where the current authentication principal is not suitable.

Additional context
None

[OlgaMaciaszek]

Hello @agileknight, thanks for reporting the issue. Makes sense.