cve-update-db-native: fix DB file version for '-'

fix logic for CVE DB update so that when the CPE version is '-',
it keeps the version as '-' in the DB file too and leave other
operation as blank.

Signed-off-by: Lee Chee Yang <chee.yang.lee@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>