Fix problem where you coudn't assign CFs to classes globally

This becomes much more obvious in 3.8.8 when we add the checkbox
in the CF admin ui rather than forcing you to navigate to Configation
-> Global -> Custom Fields

lib/RT/FM/Class_Overlay.pm

@@ -213,7 +213,7 @@ sub CurrentUserHasRight {
     return (
         $self->CurrentUser->HasRight(
             Right        => $right,
-            Object       => $self,
+            Object       => ( $self->Id ? $self : $RT::FM::System ),
             EquivObjects => [ $RT::System, $RT::FM::System ]
         )
     );