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