Fix loading follow.el

* lisp/follow.el (follow-mode-prefix-key): Avoid changing
follow-mode-map before it is defined.  (Bug#76977)
* test/lisp/follow-tests.el: New file.

lisp/follow.el

@@ -234,8 +234,9 @@ After that, changing the prefix key requires manipulating keymaps."
   :type 'string
   :set (lambda (symbol value)
          (defvar follow-mode-map) (defvar follow-mode-submap)
-         (keymap-unset follow-mode-map (symbol-value symbol))
-         (keymap-set follow-mode-map value follow-mode-submap)
+         (when (boundp 'follow-mode-map)
+           (keymap-unset follow-mode-map (symbol-value symbol))
+           (keymap-set follow-mode-map value follow-mode-submap))
          (set-default symbol value))
   :group 'follow
   :version "31.1")

test/lisp/follow-tests.el

@@ -0,0 +1,32 @@
+;;; follow-tests.el --- Tests for follow.el  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'follow)
+
+(ert-deftest follow-tests-do-nothing-test ()
+  ;; Placeholder test to verify that loading `follow' succeeds.
+  ;; Suppresses an ERT warning due to the absence of tests.
+  ;;
+  ;; Remove this test if real tests are added later.
+  )
+
+(provide 'follow-tests)
+;;; follow-tests.el ends here