systemctl: add message when edit is aborted

systemctl: add message when edit is aborted

If the user edits result in an empty file (after stripping), we would silently
not do anything, aborting the edit. This is rather confusing, let's emit a
notice:

  $ build/systemctl --user edit asdf.service --full --stdin </dev/null
  /home/zbyszek/.config/systemd/user/asdf.service: after editing, new contents are empty, not writing file.

(This also works with an editor, instead of --stdin. The message is printed on
the console and is visible after the editor exits.)

While at it, fix the condition to skip writing file after stripping. We had
"old_contents", but we modified that string. In some code flows, we would
compare the stripped old contents (i.e. a string which by defintion doesn't
have a newline at the end) with a string to which we just appended a newline
(i.e. a string which by defintion has a newline at the end).