ceph-volume: fix is_ceph_device for lvm batch

ceph-volume: fix is_ceph_device for lvm batch

This is a regression introduced by 634a709

The lvm batch command fails to prepare the OSDs on the created LV.
When using lvm batch, the LV/VG are created prior the OSD prepare.
During that creation, multiple tags are set with null value.

$ lvs -o lv_tags --noheadings
  ceph.cluster_fsid=null,ceph.osd_fsid=null,ceph.osd_id=null,ceph.type=null

Since we call is_ceph_device which returns True if the ceph.osd_id LVM
tag exists but doesn't test the value then we raise an execption.

When the tag value is set to 'null' then we can consider that the device
isn't part of the ceph cluster (because not yet prepared).

Closes: https://tracker.ceph.com/issues/44069
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>