Don't signal an error on re-builder startup with rx syntax

* lisp/emacs-lisp/re-builder.el (reb-initialize-buffer): Make
startup with rx syntax not signal an error (bug#54107).

lisp/emacs-lisp/re-builder.el

@@ -323,7 +323,10 @@ Except for Lisp syntax this is the same as `reb-regexp'.")
          (reb-lisp-mode))
         (t (reb-mode)))
   (reb-restart-font-lock)
-  (reb-do-update))
+  ;; When using `rx' syntax, the initial syntax () is invalid.  But
+  ;; don't signal an error in that case.
+  (ignore-errors
+    (reb-do-update)))
 
 (defun reb-mode-buffer-p ()
   "Return non-nil if the current buffer is a RE Builder buffer."