Prevent NPE in AuthenticationManager.backchannelLogout (#23313)

Previously, if the user was already removed from the userSession
and the log level was set to DEBUG, then an NPE was triggered by
the debug log statement during backchannelLogout.

Fixes #23306

(cherry picked from commit 04d16ed)

services/src/main/java/org/keycloak/services/managers/AuthenticationManager.java

@@ -287,7 +287,8 @@ public static BackchannelLogoutResponse backchannelLogout(KeycloakSession sessio
 
         if (logger.isDebugEnabled()) {
             UserModel user = userSession.getUser();
-            logger.debugv("Logging out: {0} ({1}) offline: {2}", user.getUsername(), userSession.getId(),
+            String username = user == null ? null : user.getUsername();
+            logger.debugv("Logging out: {0} ({1}) offline: {2}", username, userSession.getId(),
                     userSession.isOffline());
         }