tapchannel: skip creating allocations for dust balances

This commit addresses an edge case where one party only has a BTC
balance (but no asset balance). In that case the balance is allowed to
be below the dust limit. But because those sats will be added to the fee
and can't be materialized on-chain, we also can't create an allocation
for the balance. Otherwise we'll have a mismatch between the number of
allocations and the number of expected on-chain outputs of the
transaction.

This isn't an issue when there is an asset balance, as in that case we
enforce that the BTC balance is above the dust limit.

Author: guggero

tapchannel/commitment.go

@@ -969,7 +969,10 @@ func addCommitmentOutputs(chanType channeldb.ChannelType, localChanCfg,
 
 	// We've asserted that we have a non-dust BTC balance if we have an
 	// asset balance before, so we can just check the asset balance here.
-	if ourAssetBalance > 0 || ourBalance > 0 {
+	// In case there is no asset balance, and the BTC value is dust, it will
+	// not be materialized on-chain, and we shouldn't create an allocation
+	// for it.
+	if ourAssetBalance > 0 || ourBalance > localChanCfg.DustLimit {
 		toLocalScript, err := lnwallet.CommitScriptToSelf(
 			chanType, initiator, keys.ToLocalKey,
 			keys.RevocationKey, uint32(localChanCfg.CsvDelay),
@@ -1033,7 +1036,10 @@ func addCommitmentOutputs(chanType channeldb.ChannelType, localChanCfg,
 		addAllocation(allocation)
 	}
 
-	if theirAssetBalance > 0 || theirBalance > 0 {
+	// We only create an allocation if there is going to be an on-chain
+	// output. If this is dust (which is only allowed if there are no assets
+	// on the output), it will not be materialized on-chain.
+	if theirAssetBalance > 0 || theirBalance > localChanCfg.DustLimit {
 		toRemoteScript, _, err := lnwallet.CommitScriptToRemote(
 			chanType, initiator, keys.ToRemoteKey, leaseExpiry,
 			lfn.None[txscript.TapLeaf](),