Consider the point when we are at aptitude's prompt:
# aptitude full-upgrade
No packages will be installed, upgraded, or removed.
0 packages upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B of archives. After unpacking 0 B will be used.
Do you want to continue? [Y/n/?]

If we answer n, we return to the shell right away.
If we answer y, we answer to the shell after a painful grinding of
computing something that is one big no-op... Why not just detect there
is nothing to do and act like we answered n. In fact why prompt at all?
OK. In my preferences I set Always Prompt... anyway, at least bail out
fast for Y just like for n when there is nothing to do.