default Object extractAndConvert(FieldDescriptor field, Message msg)  {
      Object val = msg.hasField(field) ? msg.getField(field) : null;
      return val == null ? null : convert(val);
    }
Convert the given writable to a message.
@param writable The writable object containing the message.
@return The converted message object.
@throws SerDeException
ql/src/java/org/apache/hadoop/hive/ql/io/protobuf/ProtobufSerDe.java