Fix SASL EXTERNAL with client cert provided JID

isAnonymous() is now a bit weakened since it also considers
allowNullorEmptyUsername.

SaslExternalMechanism of smack-sasl-provided now also doesn't throw an
UnsupportedOperationException.

Follow up on 7e4e3699a1

smack-sasl-provided/src/main/java/org/jivesoftware/smack/sasl/provided/SASLExternalMechanism.java

@@ -34,7 +34,8 @@ public class SASLExternalMechanism extends SASLMechanism {
 
     @Override
     protected void authenticateInternal(CallbackHandler cbh) throws SmackException {
-        throw new UnsupportedOperationException("CallbackHandler not (yet) supported");
+        // Do nothing. Auth will be done external to Smack and which will receive the localpart
+        // after the resource binding
     }
 
     @Override