Well, today I found out that no HTLC on lightning is created for payments below the on-chain dust limit. The channel is updated so that the funds in flight go to the miner fees instead of a HTLC contract. The receiver/forwarder can't steal these money, but he can force the sender to lose them to the miner. So he doesn't have a monetary incentive to do so, but nevertheless he can force the sender to lose money.