* Force live logbook to send an empty message to indicate no results
Since the sync task can take a while if the recorder is
busy, the logbook will appear to hang if we do not send
the first partial message even if its empty.
This work is in preparation for a higher database
commit interval where this issue is most obvious.
The historical only path did not have this issue because
it never had to wait for the db sync.
* update tests
5279535046