bug#34147: 27.0.50; M-x describe-package groovy-mode breaks

[Federico Tedin]

> By evaluating describe-package-1 to get detailed Backtrace (I didn't
> paste it since it is very long), it seems the error is caused by the
> "last" expression of describe-package-1:
>
> (insert readme-string)
>
> when readme-string is nil.
>
> I notice Melpa fails to provide the readme file, this URL is 404
>
> https://melpa.org/packages/groovy-mode-readme.txt

It looks like when a package doesn't provide a 'Commentary' field, the
xyz-readme.txt file isn't generated by MELPA (see
https://github.com/melpa/package-build/blob/master/package-build.el#L382).

Other packages without 'Commentary' (in no particular order):
- instapaper
- javap-mode
- tdd-status-mode-line
- jknav
- runtests

Using C-h P on these packages will trigger the same error. I guess we
could replace:

(insert readme-string)

with:

(when readme-string
  (insert readme-string))

or:

(if readme-string
    (insert readme-string)
  (insert "This package does not provide a description."))

to cover cases where the xyz-readme.txt file for the package doesn't
exist.