Fix NPE in NASBackupProvider when no running KVM host is available

ResourceManager.findOneRandomRunningHostByHypervisor() can return null
when no KVM host in the zone has status=Up (e.g. during management
server startup, brief agent disconnections, or host state transitions).

NASBackupProvider.syncBackupStorageStats() and deleteBackup() call
host.getId() without a null check, causing a NullPointerException that
crashes the entire BackupSyncTask background job every sync interval.

This adds null checks in both methods:
- syncBackupStorageStats: log a warning and return early
- deleteBackup: throw CloudRuntimeException with a descriptive message

Author: James Peru

plugins/backup/nas/src/main/java/org/apache/cloudstack/backup/NASBackupProvider.java

@@ -478,6 +478,9 @@ public boolean deleteBackup(Backup backup, boolean forced) {
         } else {
             host = resourceManager.findOneRandomRunningHostByHypervisor(Hypervisor.HypervisorType.KVM, backup.getZoneId());
         }
+        if (host == null) {
+            throw new CloudRuntimeException(String.format("Unable to find a running KVM host in zone %d to delete backup %s", backup.getZoneId(), backup.getUuid()));
+        }
 
         DeleteBackupCommand command = new DeleteBackupCommand(backup.getExternalId(), backupRepository.getType(),
                 backupRepository.getAddress(), backupRepository.getMountOptions());
@@ -564,7 +567,14 @@ public Pair<Long, Long> getBackupStorageStats(Long zoneId) {
     @Override
     public void syncBackupStorageStats(Long zoneId) {
         final List<BackupRepository> repositories = backupRepositoryDao.listByZoneAndProvider(zoneId, getName());
+        if (CollectionUtils.isEmpty(repositories)) {
+            return;
+        }
         final Host host = resourceManager.findOneRandomRunningHostByHypervisor(Hypervisor.HypervisorType.KVM, zoneId);
+        if (host == null) {
+            logger.warn("Unable to find a running KVM host in zone {} to sync backup storage stats", zoneId);
+            return;
+        }
         for (final BackupRepository repository : repositories) {
             GetBackupStorageStatsCommand command = new GetBackupStorageStatsCommand(repository.getType(), repository.getAddress(), repository.getMountOptions());
             BackupStorageStatsAnswer answer;