transient--init-suffix: Fix regression on Emacs 28

On newer releases (kbd nil) returns "",
but when using Emacs 28 it signals an error.

Closes #5376.

Author: tarsius

lisp/transient.el

@@ -2498,7 +2498,8 @@ value.  Otherwise return CHILDREN as is.")
   (pcase-let* ((`(,class . ,args) spec)
                (cmd (plist-get args :command))
                (_ (transient--load-command-if-autoload cmd))
-               (key (kbd (plist-get args :key)))
+               (key (plist-get args :key))
+               (key (and key (kbd key)))
                (proto (and cmd (transient--suffix-prototype cmd)))
                (level (or (alist-get (cons cmd key) levels nil nil #'equal)
                           (alist-get cmd levels)