bug#67835: 30.0.50; Error with undocumented disabled commands

[kakkokakko]

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 >>"))

-- 
KH