bug#58284: 29.0.50; advice error for advised function with no docstring

[Katsumi Yamaoka]

Hi,

I found `C-h f' to not work for some advised functions.
It seems to happen when an original function has no docstring.
Here is a recipe:

(progn
  (defun foo () t)
  (defadvice foo (around testing activate) "" ad-do-it)
  (documentation 'foo))

Thanks.

In GNU Emacs 29.0.50 (build 1, x86_64-pc-cygwin, GTK+ Version
 3.22.28, cairo version 1.17.4) of 2022-10-04 built on localhost
Windowing system distributor 'The Cygwin/X Project', version 11.0.12101004
Configured using:
 'configure 'CFLAGS=-O0 -g3' --verbose --with-x-toolkit=gtk3
 --with-imagemagick'