Always schedule at least one job onto a core (#1990)

Even if the host configuration is returning `0` for the `lookahead`, we
should schedule at least one job on a core if the core exists.

polkadot/runtime/parachains/src/scheduler.rs

@@ -627,8 +627,8 @@ impl<T: Config> Pallet<T> {
 		if ValidatorGroups::<T>::decode_len().map_or(true, |l| l == 0) {
 			return
 		}
-
-		let n_lookahead = Self::claimqueue_lookahead();
+		// If there exists a core, ensure we schedule at least one job onto it.
+		let n_lookahead = Self::claimqueue_lookahead().max(1);
 		let n_session_cores = T::AssignmentProvider::session_core_count();
 		let cq = ClaimQueue::<T>::get();
 		let ttl = <configuration::Pallet<T>>::config().on_demand_ttl;