show at least something when there are no map backgrounds

zabbix ships without any map background images. when going to administration -> images -> background, it almost looks like the page failed to load completely - there's just a large grey area.
and 3.0.0 moved the version info to the bottom of the window, thus it is less apparent that there are no backgrounds.
similar as with other entities, it is suggested to add some text like "no background images found" or so.