bug#44340: 27.1; describe-text-properties: [Show] when nothing to show

[積丹尼 Dan Jacobson]

In a Gnus *Article* buffer, place the cursor on
an attachment, and do M-x describe-text-properties .

   Here is a ‘button’ button labeled ‘[4. image/jpeg; 
Screenshot_20201030-160838.jpg]...’.

   There are text properties here:
     article-type         annotation
     button               (t)
     category             default-button
     face                 gnus-button
     follow-link          t
     gnus-callback        gnus-mm-display-part
     gnus-data            [Show]
     gnus-face            t
     gnus-part            4
     help-echo            "mouse-2: toggle the MIME part; down-mouse-3: more 
options"
     keymap               [Show]
     mouse-face           highlight
     rear-nonsticky       t

OK, that's great. However clicking on [Show] just flashes the screen.
C-h e doesn't reveal why, either.
At least print a message.