bug#67835: 30.0.50; Error with undocumented disabled commands

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 15 Dec 2023 19:45:13 +0900
> From: kakkokakko@gmail.com
> 
> 1. emacs -Q --eval "(progn (defun foo () (interactive)) (put 'foo 'disabled t))"
> 2. M-x foo RET
> 
> Then, I encountered the error message "Wrong type argument:
> char-or-string-p, nil". This behavior is different from that in
> previous versions, like 27.1, where I received the following message.
> 
> | You have invoked the disabled command foo.
> | It is disabled because new users often find it confusing.
> | Here’s the first part of its description:
> | 
> |    << not documented >>
> | ...
> 
> I also expect similar behavior in 30.0.50.
> 
> In 30.0.50, it appears that disabled-command-function in
> novice.el triggers this error by executing (insert nil) for
> undocumented commands in the following code.
> 
> | (insert (condition-case ()
> |             (documentation cmd)
> |           (error "<< not documented >>")))
> 
> Although omitting a docstring is generally considered poor practice,
> triggering an error for such commands might not be intentional
> behavior. The corresponding code in 27.1 was as follows:
> 
> | (... (or (condition-case ()
> |               (documentation cmd)
> |            (error nil))
> |          "<< not documented >>"))

Stefan, could you please look into this?  The above seems to come from
your changes in commit 6ad79059d2f843a1be617c72ae0e9d8a02c9a203.  I'd
like to fix this on the emacs-29 branch soon, if possible.

Thanks.