AppleScript and Finder - How to detect an "empty" selection when copying paths to the clipboard (Expanded Question)
I think = is what you're looking for.

set sel to (get selection)
if not sel = {} then
    --stuff is selected
else
    --no stuff is selected
end if


Or remove the not and flip the cases

set sel to (get selection)
if sel = {} then
    --no stuff is selected
else
    --stuff is selected
end if