Fix aggregator flush method

When no samples have been handled by the aggregator but flush method is
called, an empty array must be returned because 'initial_timestamp' have
not been set yet.

Also, the pipeline always catchs exception from transformer and when an
error occurs into a transformer the unit can not catch them.
The patch also reraises any transformer exceptions.

Closes-bug: #1325507
Co-Authored-By: Ala Rezmerita <ala.rezmerita@cloudwatt.com>

Change-Id: Ib3183dfa3ee37ba586eb03a73f5de4641dd98514
Merge "Fix aggregator flush method"