webadmin: fix error when changing cluster of diskless vms

DisksAllocationView#addDiskList wasn't prepared for getting a model
without disks sets (i.e., 'disks' is null) and since 50e0bb9 this
might happen, therefore proper handling (treating this case as if an
empty list of disks is set) is added to DisksAllocationView#addDiskList.

Bug-Url: https://bugzilla.redhat.com/2121083
Signed-off-by: Arik Hadas <ahadas@redhat.com>

frontend/webadmin/modules/gwt-common/src/main/java/org/ovirt/engine/ui/common/widget/uicommon/storage/DisksAllocationView.java

@@ -1,6 +1,8 @@
 package org.ovirt.engine.ui.common.widget.uicommon.storage;
 
 import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
 
 import org.ovirt.engine.ui.common.CommonApplicationConstants;
 import org.ovirt.engine.ui.common.PopupSimpleTableResources;
@@ -189,11 +191,15 @@ private void initListeners(final DisksAllocationModel model) {
 
     void addDiskList(DisksAllocationModel model) {
         diskListPanel.clear();
-        diskAllocationLabel.setVisible(!model.getDisks().isEmpty());
+        List<DiskModel> disks = model.getDisks();
+        if (disks == null) {
+            disks = Collections.emptyList();
+        }
+        diskAllocationLabel.setVisible(!disks.isEmpty());
 
         int diskIndex = 0;
         String columnWidth = calculateColumnWidthPercentage();
-        for (final DiskModel diskModel : model.getDisks()) {
+        for (final DiskModel diskModel : disks) {
             DisksAllocationItemView disksAllocationItemView = new DisksAllocationItemView(columnWidth);
             disksAllocationItemView.edit(diskModel);
             disksAllocationItemView.setIsAliasChangeable(model.getIsAliasChangeable());