The cron module forces changed=True when there was no real change,
but the original crontab did not contain a final newline, which is
mandatory.
When the user has no crontab or the user does not exist at all,
crontab -l exits with 1 and the cron module correctly interprets
this as "no crontab" and stores the old crontab as "".
However this triggers changed=True, even if we're not going to
change anything, e.g. when removing a crontab entry from a user
who has no crontabs at all.
Let's special-case the fact that the old crontab is empty and not
force changed=True in that case.
61579aebb2