bug#52926: dired-copy-filename-as-kill should beep if not on a file line

[積丹尼 Dan Jacobson]

 C-x d                        ;; dired-at-point
 <return>                     ;; exit-minibuffer
 <up>                         ;; dired-previous-line
 <up>                         ;; dired-previous-line

Now I am on the line that says
"total used in directory 198228 available 3.5 GiB"

 w                            ;; dired-copy-filename-as-kill
 C-x b                        ;; switch-to-buffer
 b                            ;; self-insert-command
 <return>                     ;; minibuffer-complete-and-exit
 C-y                          ;; yank

There is nothing to yank.

emacs-version "27.1"