Fix: consumer data pending for empty streams

Fix: consumer data pending for empty streams

We should at least output one packet before a stream can be considered
as readable. So far, for PID buffers, if an application exits at the
wrong timing before a stop waiting for data pending, empty streams could
be visible by a babeltrace executed after data pending incorrectly
returned false.

Fix it by considering a stream for which the consumerd has written 0
bytes to the output as having data pending.

This applies to 2.3-rc and stable-2.2.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>