zed: Avoid core dump if wholedisk property does not exist

zed aborts and dumps core in vdev_whole_disk_from_config() if
wholedisk property does not exist. make_leaf_vdev() adds the
property but there may be already pools that don't have the
wholedisk in the label.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #14062