Principal proxy handler fails when not authenticated

When current request is not authenticated, the Proxy handler for Principal fails with a NullPointerException. We should check if doGetPrincipal is null and return null straight instead of trying to invoke the target method on a null object.

Caused by: java.lang.NullPointerException
at sun.reflect.GeneratedMethodAccessor114.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.openejb.cdi.ManagedSecurityService$1.invoke(ManagedSecurityService.java:83)