consumerd: on_sleep not called on stream when no data is available

consumerd: on_sleep not called on stream when no data is available

The `on_sleep` stream operation is not invoked when a stream's `get`
operation returns ENODATA (no data available).

Since this is not an error, the normal steps of the consumption loops
should be assumed.

Not marked as a fix as this is not problematic right now. However, it
could prove misleading in the future.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I0812e3af4c967390ebba4128781787abf45c76a1