code: @SuppressWarnings("unchecked")
      protected Map<String, byte[]> documentToBytesMap(Map<String, Object> doc) {
          if (doc == null || doc.size() == 0) {
              return Collections.EMPTY_MAP;
          }
          Map<String, byte[]> data = new HashMap<>();
          doc.forEach((k, v) -> {
              byte[] value = SerializationUtils.toByteArrayFst(v);
              if (value != null) {
                  data.put(k, value);
              }
          });
          return data;
      }
docs: Sereialize "document" to byte-array map.
  
  @param doc
  @return