xl: Tree walking should not quit when one disk returns empty (#9160)

Currently, a tree walking, needed to a list objects in a specific
set quits listing as long as it finds no entries in a disk, which
is wrong.

This affected background healing, because the latter is using
tree walk directly. If one object does not exist in the first
disk for example, it will be seemed like the object does not
exist at all and no healing work is needed.

This commit fixes the behavior.

cmd/xl-v1-list-objects.go

@@ -34,14 +34,10 @@ func listDirFactory(ctx context.Context, disks ...StorageAPI) ListDirFunc {
 			var newEntries []string
 			var err error
 			entries, err = disk.ListDir(bucket, prefixDir, -1, xlMetaJSONFile)
-			if err != nil {
+			if err != nil || len(entries) == 0 {
 				continue
 			}
 
-			if len(entries) == 0 {
-				return true, nil
-			}
-
 			// Find elements in entries which are not in mergedEntries
 			for _, entry := range entries {
 				idx := sort.SearchStrings(mergedEntries, entry)
@@ -58,6 +54,11 @@ func listDirFactory(ctx context.Context, disks ...StorageAPI) ListDirFunc {
 				sort.Strings(mergedEntries)
 			}
 		}
+
+		if len(mergedEntries) == 0 {
+			return true, nil
+		}
+
 		return false, filterMatchingPrefix(mergedEntries, prefixEntry)
 	}
 	return listDir