[#207] Fix list parts with empty list

Signed-off-by: Marina Biryukova <m.biryukova@yadro.com>

api/layer/multipart_upload.go

@@ -548,7 +548,7 @@ func (n *layer) ListParts(ctx context.Context, p *ListPartsParams) (*ListPartsIn
 		return parts[i].PartNumber < parts[j].PartNumber
 	})
 
-	if p.PartNumberMarker >= parts[len(parts)-1].PartNumber {
+	if len(parts) == 0 || p.PartNumberMarker >= parts[len(parts)-1].PartNumber {
 		res.Parts = make([]*Part, 0)
 		return &res, nil
 	}