When user creation is blocked, we get a defined CurrentUser

RT-Authen-ExternalAuth can block user creation in the mailgate by
returning 0 from CanonicalizeUserInfo if the user doesn't exist in the
external system.  RT blindly assumes that
RT::Interface::Email::CreateUser will return a user, so it returns the
CurrentUser and 1 without checking to see if the creation was
successful.

This results in CommandByMail getting a defined but unloaded CurrentUser

Checking for the Id prevents later failures

lib/RT/Interface/Email/Filter/TakeAction.pm

@@ -148,7 +148,7 @@ sub GetCurrentUser {
         @_
     );
 
-    unless ( $args{'CurrentUser'} ) {
+    unless ( $args{'CurrentUser'} && $args{'CurrentUser'}->Id ) {
         $RT::Logger->error(
             "Filter::TakeAction executed when "
             ."CurrentUser (actor) is not authorized. "