Open a streamed block for transactional messages during decoding

Hi,

While reviewing the test_decoding code, I noticed that when skip_empty_xacts
option is specified, it doesn't open the streaming block( e.g.
pg_output_stream_start) before streaming the transactional MESSAGE even if it's
the first change in a streaming block.

It looks inconsistent with what we do when streaming DML
changes(e.g. pg_decode_stream_change()).

Here is a small patch to open the stream block in this case.

Best Regards,
Hou Zhijie