<fix>[kvm]: fix NPE in VmHostFileTracker sync

When all getLastSyncDate() values in the stream are null, the
min() operation with Comparator.nullsFirst() throws NPE
because Optional.of() doesn't accept null. Filter null values
before calling min() to avoid this issue.

Resolves: ZSV-11779
Related: ZSV-11310

Change-Id: I646a65776a717465636e75616c70746763727862

Author: Zhang Wenhao

plugin/kvm/src/main/java/org/zstack/kvm/vmfiles/VmHostFileTracker.java

@@ -269,7 +269,8 @@ private void syncVmHostFiles() {
                 // check if force sync is needed based on lastSyncDate
                 Timestamp oldestLastSync = group.stream()
                         .map(VmHostFileVO::getLastSyncDate)
-                        .min(Comparator.nullsFirst(Comparator.naturalOrder()))
+                        .filter(Objects::nonNull)
+                        .min(Comparator.naturalOrder())
                         .orElse(null);
 
                 if (oldestLastSync != null && (now - oldestLastSync.getTime()) < forceSyncThresholdMs) {