bug#34196: Error in auto-revert-buffers: Wrong type argument

[Alex Branham]

Hi Michael -

The fix for this bug seems to cause issues if default-directory is nil
for any buffer. In particular, you get a wrong type argument where
auto-revert-buffers is expecting a string instead of nil.

This change fixes the issue for me:

Thanks,
Alex

diff --git a/lisp/autorevert.el b/lisp/autorevert.el
index 150693baf1..318b10d4a8 100644
--- a/lisp/autorevert.el
+++ b/lisp/autorevert.el
@@ -773,7 +773,8 @@ the timer when no buffers need to be checked."
                        (mapcar
                         (lambda (buf)
                           (with-current-buffer buf
-                            (and (or (not (file-remote-p default-directory))
+                            (and default-directory
+                                 (or (not (file-remote-p default-directory))
                                      (file-remote-p default-directory nil t))
                                  buf)))
                         bufs)))