webadmin: Fix NPE in Edit Pool Dialog

Creation of the dialog was failing due to NPE in
DisksAllocationModel.sortDisks() when the list of disks was null.

Added relevant null-check.

Change-Id: I83e3d1308519d1abb49cd31a3fe3298d517e7aa6
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=959936
Signed-off-by: Martin Betak <mbetak@redhat.com>