fix(restore): handle download of fully versioned batch

Recent commits changed versioned batch download so that
if any sstable component is versioned, then all sstable components
are downloaded as versioned files.
It was done in that way to allow easier versioned progress calculation
(we don't store per file size, only the whole sstable size).

This brought to light a bug (that existed before, but was more difficult to hit),
in which restoring batch failed when the whole batch was versioned,
as calling RcloneSyncCopyPaths on empty paths parameter resulted in broken download.

We could just skip the RcloneSyncCopyPaths call when the whole batch is versioned,
but this would leave us without the agentJobID which is a part of sort key
in RestoreRunProgress. Without it, we could potentially overwrite one
restore run progress with another - if both of them happened on the same
RemoteSSTableDir, by the same Host, and were fully versioned.
It would also introduce a different path for restoring regular batch and fully versioned batch,
which is not desirable.

That's why I decided to modify rclone server to allow empty path parameter,
so that it still generates agentJobID, but it doesn't do anything except for that.

pkg/rclone/rcserver/rc.go

@@ -554,7 +554,9 @@ func rcCopyPaths() func(ctx context.Context, in rc.Params) (rc.Params, error) {
 		if err != nil {
 			return nil, err
 		}
-
+		if len(paths) == 0 {
+			return nil, nil
+		}
 		return nil, sync.CopyPaths(ctx, dstFs, dstRemote, srcFs, srcRemote, paths, false)
 	}
 }

pkg/scyllaclient/client_rclone.go

@@ -280,6 +280,9 @@ func (c *Client) RcloneCopyPaths(ctx context.Context, host, dstRemoteDir, srcRem
 	if err != nil {
 		return 0, err
 	}
+	if paths == nil {
+		paths = make([]string, 0)
+	}
 
 	p := operations.SyncCopyPathsParams{
 		Context: forceHost(ctx, host),