webadmin: NPE on PoolListModel

The
1cf05c520c3e30f68ac12b5772a251aa78d46bbd
did bring a regression because it expected that the
disks can not be null, but they actually can be.

Added a null check.

Change-Id: I9f26ef2ab847cfec13bf13852e7c0647889183cb
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>