Fix apt_has_updates fact not parsing apt-check output correctly

The /usr/lib/update-notifier/apt-check script returns its output
to STDERR but a recent change to the script redirects STDERR to
/dev/null. This will cause the array to always be empty.

Combined with that problem, while we were checking for the result
being nil, we never checked for an invalid array. As a result,
the apt_has_updates was always true and the apt_updates and
apt_security_updates facts were trying to read from an empty array
and failing.