Missing nullable types for Message stub

[jahsen]

We saw that when there is an error, and a message is consumed, there is possibility
to have null in message topic_name and message timestamp properties.
The exact error is -185 / RD_KAFKA_RESP_ERR__TIMED_OUT.
Probably it is a good idea to make them null-able. :)

[kwn]

Thanks for flagging the issue. Do you mind opening a pull request and adding the missing value?

[jahsen]

Of course :)