cve-update: handle baseMetricV2 as optional
Currently in NVD DB an item popped up, which hasn't set baseMetricV2. Let the parser handle it as an optional item. In case use baseMetricV2 before baseMetricV3 Signed-off-by: Konrad Weihmann <kweihmann@outlook.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>