cve-check: write empty fragment files in the text mode

In the cve-check text mode output, we didn't write fragment
files if there are no CVEs (if CVE_CHECK_REPORT_PATCHED is 1),
or no unpached CVEs otherwise.

However, in a system after multiple builds,
cve_check_write_rootfs_manifest might find older files and use
them as current, what leads to incorrect reporting.

Fix it by always writing a fragment file, even if empty.

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit f1b7877a)
Signed-off-by: Steve Sakoman <steve@sakoman.com>