LSPS2: Lower-bound amount to forward by HTLC min.

[tnull]

In lightningdevkit/lightning-liquidity#56 we concluded the forwarded amount need to be lower-bounded by htlc_minimum_msat/ChannelDetails::next_outbound_htlc_minimum as currently the fee calculation could result in some 0-amount HTLCs under certain circumstances. We then also should check that in the corresponding test.