Fix: src.ctf.lttng-live: consider empty metadata packet as retry

When clearing a live session, it's possible for the Relay to know that
metadata is available even though it has not yet received it from the
consumer.

In such cases, the relay will send a `LTTNG_VIEWER_METADATA_OK` reply
with a zero length metadata packet. The viewer needs to interpret that
as "please try again later". In fact, the viewer needs to keep
requesting metadata from the relay until it receives either a
`LTTNG_VIEWER_NO_NEW_METADATA` status or an error status.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ief33f1a43ec827e211a2f4f2ace623e9d3254246
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7392