webadmin: null safe StorageDomainFreeSpaceRenderer

StorageDomainFreeSpaceRenderer:
verify getAvailableDiskSize's value before rendering.

Change-Id: Ica5bd15bb939ec86502e406dcdd56f4dcf604ac7
Bug-Url: https://bugzilla.redhat.com/1079710
Bug-Url: https://bugzilla.redhat.com/1132406
Signed-off-by: Daniel Erez <derez@redhat.com>