process_pipeline_inlet_filter() and its outlet counterpart construct and tear
down an aiohttp ClientSession, with its own connector and cookie jar, on
every chat completion and every task generation request just to iterate an
empty filter list. On deployments without pipelines, which is the default,
that is wasted setup on every message.
Both functions now return the payload untouched before the session is
created when there is nothing to call. The per-call saving is small, a few
microseconds of object construction per request on the pinned aiohttp; the
point is that requests stop paying setup for a feature that is not
configured.