FreeBSD: Fix potential boot panic with bad label

vdev_geom_read_pool_label() can leave NULL in configs. Check for it
and skip consistently when generating rootconf.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #14291
(cherry picked from commit dc8c2f615852cb79d3c4cae6c1fb738c7f4a793c)