sc-consensus-beefy: fix on-demand async state machine (#13599)

`futures_util::pending!()` macro only yields to the event loop once,
resulting in many calls to the `OnDemandJustificationsEngine::next()`
made by the tokio reactor even though the on-demand-engine is idle.

Replace it with the correct `futures::future::pending::<()>()` function
which forever yields to the event loop, which is what we want when
the engine is idle.

Signed-off-by: Adrian Catangiu <adrian@parity.io>