transient--make-predicate-map: Fix detecting outer prefix

Since [1: 5f2cfc9] we try to detect whether there is an outer
prefix, that non-transient suffixes could return to, by checking
if `transient--current-prefix' is non-nil.  That works then the
prefix is created, but that variable is also always non-nil after
a transient suffix is invoked.  Instead check if `transient--stack'
is non-nil.

1: 2023-11-29 5f2cfc9
   transient--make-predicate-map: Only return if there is a parent

lisp/transient.el

@@ -1831,7 +1831,7 @@ of the corresponding object."
 (defun transient--make-predicate-map ()
   (let* ((default (transient--resolve-pre-command
                    (oref transient--prefix transient-suffix)))
-         (return (and transient-current-prefix (eq default t)))
+         (return (and transient--stack (eq default t)))
          (map (make-sparse-keymap)))
     (set-keymap-parent map transient-predicate-map)
     (when (or (and (slot-boundp transient--prefix 'transient-switch-frame)