support/scripts/pkg-stats: fix the status reporting of CVEs

Since commit bd665d18
("support/scripts/pkg-stats: improve rendering of CVE information"),
we have better reporting of CVE related information, based on
pkg.status['cve']. However, this commit broke pkg-stats when the
--nvd-path option is not passed, and therefore no CVE information is
available.

This commit fixes that, by making use of the is_status_ok(),
is_status_error() and is_status_na() methods recently introduced.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>