transient--make-predicate-map: Only return if there is a parent

Only prepare to return to the parent transient if there actually
is a parent.  The only negative effect of failing to do this was
that the suffix was colored wrong, since `transient--do-return'
falls back to behave like `transient--do-exit'.

lisp/transient.el

@@ -1810,7 +1810,7 @@ of the corresponding object."
 (defun transient--make-predicate-map ()
   (let* ((default (transient--resolve-pre-command
                    (oref transient--prefix transient-suffix)))
-         (return (eq default t))
+         (return (and transient-current-prefix (eq default t)))
          (map (make-sparse-keymap)))
     (set-keymap-parent map transient-predicate-map)
     (when (or (and (slot-boundp transient--prefix 'transient-switch-frame)