> If there are no differences then emacs should just show this in a message instead of popping up a window with not useful content.

Just a simple implementation with advices for the time being for those who prefer this:

(advice-add 'diff :around 'my-diff)

(defun my-diff (orig-fun &rest args)
(flet ((display-buffer (buf) buf))
(apply orig-fun args)))

(advice-add 'diff-sentinel :after 'my-diff-sentinel)

(defun my-diff-sentinel (code &rest args)
(if (equal code 0)
(message "No differences.")
(display-buffer (current-buffer))))