extensions-window: Minor UI tweaks to the "No Results Found" page

extensions-window: Minor UI tweaks to the "No Results Found" page

In many GNOME apps, the "No Results Found" status page use
edit-find-symbolic as the icon, so use it in Extensions too
to keep consistency.

Also, remove the period from the status page description, as
per HIG writing style guidelines[1].

[1] https://developer.gnome.org/hig/guidelines/writing-style.html#periods

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3270>